Well, maybe not a cake--more like a hot pink puffed rice cookie...Anyway, here's a nice way to make a mould for any pourable confection that doesn't require baking.
Step 1: Draw your footprint.
If you have a computer file, print it out. If you want to draw freehand, do it. I wanted to use the MAKE Magazine logo to celebrate our first birthday, and decided to try drawing it freehand.
I used a waxed paper box as the basis for my line width. I had the logo on a magazine cover as reference, and used a ruler to help me judge the width and height of each letter in reference to each other letter. It's not a perfect rendition, but it's pretty close.
Step 2: Make the walls of the mould.
Using the letters (or images) you've drawn as a footprint, make walls to support the material you're pouring in (in my case, rice crispy treat batter). I used the walls of the pizza box, but any long strip of cardboard will do. You will want to make sure the strips are even in height, and that the corrugation of the cardboard is vertical for horizontal strips. This makes it easy to bend your pieces to fit around curves--each bit of corrugation will bend in the right direction. If your corrugation goes the long way, then your bends will be hard to keep straight.
Tape the long pieces of cardboard together if each individual piece is not long enough for the footprint shape, and tape down to the footprint on the outside for stability.
Step 3: Grease the mould.
Butter (or margarine) probably works best as it won't soak through the cardboard. This keeps the rice crispy treat batter from sticking and makes removing the mould easy.