They are time-consuming and require a lot of steps which I spread over about two days, but the reception they got made it worth every minute. This instructable will tell you how I made them.
Step 1: Planning & Ingredients
1. Cake. You will need a sheet cake, which I made using King Arthur Flour's Elegant White Cake recipe, with the only change being the substitution of virgin coconut oil (which I had in the pantry) for the vegetable shortening (which I think is gross). I don't buy cake flour, but make it like this with much success.
2. Pastry cream (or frosting). Since my cake recipe calls for five egg whites, I elected to make pastry cream (instead of frosting), which requires five yolks. The Brown-Eyed Baker's recipe is excellent and hasn't failed me yet. Pastry cream is super fun to make, not overly sweet, and marvelously smooth. But you can use frosting if you prefer. You will have extra whichever route you choose; with pastry cream, you can make cream puffs or fruit tarts. If you're a frosting fan, I guess you can just eat it with a spoon.
3. Fruit. I used raspberries, a kiwi, some mango, and a peach.
4. Chocolate. Rather a lot of it, like possibly most of a bag of nice quality chips.
5. Shredded coconut, sweetened or unsweetened according to your preference (I go with unsweetened).
6. Glaze. Mine required the juice of one lemon, about a quarter cup of brown sugar, and a dash of dark rum. The glaze keeps your cut fruit from browning, so don't skip it.
1. Parchment paper
2. A half-sheet pan (if you're making the cake) and sauce pan (for the pastry cream) and bowls and all that
3. A small biscuit cutter
4. A small sauce pan for the glaze and a brush
5. Surfaces to put your sushi on going in and out of the fridge, and something to cover them with
6. A chocolate-melting apparatus, such as a double-boiler or a dish that sits on a sauce pan
7. A round pastry tip with coupling (pastry tips are cheap to buy individually, fun to use, and they take up almost no storage space so there's no reason you shouldn't have a couple) and a brand-name zip-top bag (the cheap-o ones will burst under the pressure of the cream - this has happened to me; just buy the good bags). Snip the corner from the zip bag and attach your pastry coupling to it.
Step 2: Cutting the Cake
With the remainder of the cake (about a third, in this case), use a big ol' serrated cake or bread knife to carefully cut rectangles about an inch wide and two and half inches long. Gently squeeze each rectangular piece in your hand as though you were pressing a handful of rice together, to soften the hard-edged look. Go slowly to keep from crumbling them.
Step 3: Chocolate Coating
You want to keep your cake pieces covered and stashed in the fridge or freezer whenever you are not working on them. They are much easier to handle when frozen.
Melt some chocolate. Take your rounds out of cold storage and coat them in chocolate by touching the edge to the surface of the chocolate and slowly spinning the round while holding it with two fingers in the middle, leaving the flat edges uncoated. Carefully set the each piece back on the parchment-lined tray and put them back in the freezer to set.
Note: I tried doing a few of them with Scoochmaroo's magic shell coating, but I regretted it because it meant those pieces had to be refrigerated permanently. Using pure chocolate means the finished pieces can survive at room temperature, and they are also darker, which I preferred for the obvious reason of greater realism.
Step 4: Coconut Coating
Toast the coconut lightly in the oven at 350 for a few minutes to enrich the flavor and improve its texture. It will brown a little bit; take it out before it browns a lot.
While that happens, let the rectangles thaw on the counter. Their outsides will become moist, and the coconut will stick. The tops and bottoms don't need to be coated.
Step 5: Pastry Cream Filling
For the rounds: I poked holes in the center of each piece, and my initial idea was for the pastry cream to be invisible and only serve to anchor the piece of fruit to the cake. However, the pastry cream is delicious and its moisture is essential to the overall pleasure quotient of each bite; much more was needed than I could squeeze into that little hole. Covering the entire top surface was much more satisfactory. I still advise poking the hole, though. Use the blunt end of a chopstick or anything of a similar shape. Pipe the pastry cream into the hole and onto the top of each round.
For the rectangles: pipe the pastry cream over the entire top surface of each rectangle.
At this point, do not store them in the freezer any more. I make no claims about what pastry cream will or will not do if frozen.
Step 6: Fruit on Top
Apply fruit. I used raspberries (delicious, and resemble roe) and kiwi for the rounds, and slices of mango and peeled peach for the rectangles. (I ate the peel.)
Step 7: Glaze Cut Fruit
To make the glaze, heat the juice of a large lemon, a quarter cup of brown sugar, and a dash of rum (or vanilla) in a small sauce pan over medium-low heat. Let it simmer very gently while you stir it (and clean up and do other stuff) for a few minutes, until it starts to look viscous. It doesn't thicken much - that's fine. It will thicken if you take it off the heat, though, so leave it on low until you're finished glazing.
Brush the glaze onto the exposed cut peaches and mangoes.
Step 8: Package and Bestow (or Eat)
I bought this cake box from a local bakery. It was deep, so I made a shelf using pieces of paper towel tubing (including a piece in the middle) and a trimmed sheet of thin cardboard covered with parchment, so I could fit two layers of sushi inside.
Voilà! Cake sushi. Every bit as cute as raw fish, and safer to give as a birthday present.