This is an overview of how I made a nyancat food-safe mold, and of course with the magic of CNC and digital CAD files you can apply it to just about any shape you want. The basic idea is to mill out a foam positive, pour food-safe silicone into that, and then when the silicone mold cures, pour food into that.
CAD of some sort. Autodesk inventor is free for students (people with .edu emails).
Access to CAM and CNC mill or 3d printer.
Some money to buy food-safe silicone, say $20. Well, if you're doing jello, you can make molds out of hot glue (I used that for https://www.instructables.com/id/LeGummies-brick-shaped-gummy-candies/ -- definitely a destructive mold removal, I had to pry legos out with pliers), but I'm not 100% sure how safe that is. I guess it'd be fine for things like pudding.
Cake / pudding / jello mix, yogurt, watermelon juice, whatever food item you want.
1) Spend a few hours CAD'ing nyancat. Add a shell to the CAD file to make a positive mold shape you can pour silicone into
3) CNC mill out of architecture foam or 3d print the positive mold
4) Apply 2-part food-safe silicone (putty or rubber) to the positive mold to make a negative mold
5) Apply cake mix or jello or pudding or whatever. If you're making cake, make sure to use mold release, aka
Submitted by MITERS for the Instructables Sponsorship Program
Step 1: NyanCAD! Yay! *dance*
Images are marked with  below.
The first step is to CAD whatever you want to make a mold of. You CAD a positive version of your image, then add a "shell" around it so that you can pour silicone into the shape.  The shell doesn't have to be circular, I just made it that way because originally I wanted a cupcake.
There are three main considerations when designing your mold.
If you are milling out your mold:
1) what size drill bit decide to use limits what size features you can have (but for cake, small features don't show up well anyway)
2) the length of the drill bit you are using limit how many levels / how deep you can go
3) the size of the mold impacts how much of that expensive food-safe silicone you will use
What I did:
You can see how I started a sketch and then used tools > sketch tools > sketch picture.... to insert a reference nyancat image into my solidworks file.  
Since nyancat is pixelated, I also played around with the grid settings to get a grid that roughly matched the image (as well as scaling the image to get it to match).  Specifically, I knew I wanted to use a 1/8'' bit to get the file details without spending forever (using a 1/16th bit), so the sprinkles are a bit larger than that. I used a grid size of 0.14'', and the mold is about 6'' in diameter. This is huge! It eats up almost the full $20 of silicone mold-making material I got off of amazon. More about this in the 3rd step.
I hadn't really considered the depth of the mold, and I was lucky in that the drill bit I used was just deep enough to cut the grooves.
The drill bit definitely wasn't long enough to cut out the mold -- I was watching out for this and stopped it just as it started to bury itself into the foam. 
Note that if you're planning on making cakes, finer details won't show up. e.g. the sprinkles on this CAD model definitely don't show up in cake. Actually, they tend to show up as reverse-sprinkles.  It shows up all fine on the jello and pudding molds though.
For milling, I didn't want to waste time having the mill cut out a circular shape, so in the last picture  you can see that I made the shell extra-large so that it wouldn't attempt to mill that surface, and then when I was using the mill's CAM software, I selected a rectangle around the shape to cut out the mold.
Finally, I've attached my CAD files here:
1) the sldprt (solidworks 2013*) file and
2) the exported STL file that CAM programs for the mill or 3d printer can read in to generate x,y,z movements (or polar? I know people with homebrew robot arm and delta robot 3d printers...) for the motors to move the cutting/extruding end around.
*ugh, I really hate how they are not backwards compatible, e.g. you can't open it in solidworks 2012 even. I'm going to graduate and not being able to use any of these files -__-;
for my ease of editing, the folders I am using:
Step 2: Mill / 3d Print It
For milling, you can use insulation foam, which is cheap and easy to mill (also used extensively in architecture models). The foam is nice because the silicone releases easily from it. Just don't breathe too much of the mill dust in.
It's sold in stores as "foam insulation" or "extruded polystyrene." See the internet for more information.
A note about 3d printing the mold, the outer "shell" tends to come off when I 3d print a scaled version of my 6'' diameter nyanCAD, which can be fixed with a dab of superglue.
Cleaning the foam mold
At the end, you can take a hot air gun to clean the foam "burrs" leftover from milling (see pic 10/23 14:07). This melts the polystyrene a bit (see picture from 10/23 15:34), so it wasn't really suitable for pixelated nyancat with sharp corners; I just manually picked off most of the bits of foam.
I was using the shopbot in the MIT media lab, so I used Partworks. I've also used masterCAM, where you have to draw the toolpaths by hand. This is actually insanely hard to do with nyancat because all the paths end up in the same place and MasterCAM gets all confused. Ugh. Get some UX people on that software...
Partworks was just like "hey look! it's a 3d file! let me magic that file for you! Let's be friends!"
I took pics of some of the settings I used for partworks. I cut two molds, one in december and one in november, which is why there's two sets. They are all the pics at the end.
You can also see a small 3d print, I was fixing up hexarideablepod (based off of https://www.instructables.com/id/Hexabot-Build-a-heavy-duty-six-legged-robot/) at the time for NYC maker faire 2012, so I didn't have time to make the mold walls properly thin. I actually got some super nice people at nyc maker faire printing pavilion to print off a mold with thinner walls, but had to leave before it was finished (looks like the printer need some setting adjustments as well). Thanks Luc Nikiema and the Tjiko snap 3d people. The plan was to ask the people printing pancakes for some batter and enjoy some nyan-pancakes :)
Note:molds are not mice-proof! Make sure to store your mold in a mouse-proof location! I left mine at school over winter break and came back to see my mold destroyed by mice chewing on them. WTF, seriously, not cool, mice :'(
Step 3: Pour Silicone Mix
I bought mine off of amazon prime because I had the student trial, so if you're shorter on time than money you can get it off of amazon.
1lb Easymold Silicone Rubber
Although, holy crap, it was $26.52 when I bought it, it's now at a whopping $34.63. You can see the price changes over time on camelcamelcamel:
So, if you have the time to wait for shipping, maybe hunt around the internet instead, or perhaps your local crafts store.
There's some at http://www.makeyourownmolds.com/copy-flex for $25 a lb, but it takes another $10 of shiping so @__@
Smooth-on, the makers of everything mold-related, also has food-safe silicone: "Smooth-Sil® 940 , Sorta Clear® 40, Sorta Clear® 18 and the Equinox® Series." I looked at the MA distributor, http://www.smooth-on.com/pages.php?pID=104 > Reynoldsam.com. Looks like that price is slightly more reasonable, running at ~$35 for two lbs, except plus shipping... There's no real good answer here, except get sponsored by arts grants at your university or city or something :P
The putty is here:
1lb Easymold Silicone Putty
How much to use
I poured rice into my foam mold to estimate how much mix I would need (water could be used as well, it's just a bit messier), then poured it out into a tupperware. If you just have one plastic container (the silicone will peel off of this when it is cured), you can mark the line with a sharpie and dump the rice out. I just used another identical tupperware container.
The easymold mix is 1:1 part a:part b by volume, so it was pretty straightforward to pour halfway with part A and then the rest of the way with part B. Then, I used popsicle sticks to mix it up.
Saving on the rubber
I stuck some foam bits I carved out by hand onto a piece of a cardboard to make a sort-of-two-piece mold to try to save a bit of material. If you do this, you should account for this in the rice-estimating step.
I clamped the top on and let it sit for half an hour, then removed it.
Removing the mold/ cleaning the silicone mold
The silicone should come cleanly off of the foam positive once its cured. I end up destructively removing the mold, but if you design your mold well you shouldn't need to really destroy the foam positive. I didn't deburr/clean out the foam mold enough, so bits of foam mold got stuck in the rubber mold. Some bits are probably inevitable. I just picked away at the silicone mold for a bit. You could probably also melt it out, the MSDS says the combustion products are CO, CO2, and soot. "polystyrene msds" or there is probably an MSDS (materials safety data sheet) for the specific insulation product you end up using.
It does NOT like coming off of cardboard as much.
The silicone comes off of the 3d print ABS extremely cleanly. The silicone captures extremely fine details, so it will capture the "3d printed" strands of plastic look.
Step 4: Apply Food!
Make sure to apply vegetable oil or butter! The face is especially difficult, so watch out for that. The failure modes for nyancake are plentiful. Your cake needs to be well-done, and if you don't apply mold release (oil/butter) you get zombie nyancakes or faceless nyancakes.
You can see that the mold heats unevenly -- if the temperature is up high, I think this was for white cake, the top gets burnt. Mmm, food visualization of thermal distribution...
The 1/23/12 picture shows how much one box of cake mix makes -- enough to fill 2.5 nyancake molds, probably.
If you're lazy, you can use the dieter's trick and all you need is cake mix and a clear soda. The clear soda replaces all the liquid ingredients. This version actually seems to distribute heat better, or perhaps it depends on the type of cake. To be determined! (science?)
From jell-o mix. It's all hunky-dory compared to cake, but still involves heating up water. It captures much finer details than cake, so
Whoa! All you have to do is add milk to the mix and whisk it for two minutes and you're done. It's a lot more wobbly than jello when not-so-frozen though.
I haven't gotten this to work well for anything except the smallest1'' diameter mold.The silicone is somewhat of an insulator and does not heat through fast, so you can't really flip your pancake and kind of just have to sit there and wait a minute or two.
The laziest way! Just dump some yogurt in there and stick it in the freezer :)
Bread / cookies / muffins:
I haven't tried this yet, but seems like muffin bread (or a thicker kind of cake) would capture the details of the mold better than cake.
Step 5: Conclusion
Reducing the amount of material probably requires making a two-part mold, like what I did with the cardboard but more... CNC. Something like this: http://fablab.waag.org/archive/casting-foosbal-man-part-2.
I've also shifted toward small 3d-prints in the hopes of making a nyancupcake tray one day. Haven't gotten around to it yet though.
Changing the material
For oven-safe materials, it seems like silicone rubber is it. I've considered making a negative out of metal or heat-resistant plastic and then coating it with "silicone food sealant," however that stock is expensive too. If I knew more about woods, perhaps there are some suitable for use in ovens at <400 degrees Fahrenheit.
It's possible I could mill a chunk of sugar candy or something, oil it really thoroughly, and then pour cake batter into that. It wouldn't really be reusable though.
PET-G and corn starch have also been used to make food molds, although the former is not suitable for the oven and the latter is one-time use. See https://www.instructables.com/id/Custom-Hamburger-mold and https://www.instructables.com/id/Corn-Starch-Candy-Molds.
Singing nyancake! Embed an attiny+piezo+battery inside the cake, perhaps add some code so only when it is picked up does it start singing. See: jarv.org/2011/11/custom-musical-greeting-card-for-less-than-5/.
Hmm, valentine's day is coming up. There may be some mold-making in my future...
(for my editing use, my blog entries on this topic: drystonecakemakerfairenyancad)