Hi Eggy :-) My second graders read this story but I have never made the cake. I am sure it will be a good recipe. Just make sure to puree the tomatoes very finely in a blender or food processor. Here are 2 different recipes for Chocolate Buttercream Frosting. The first is from the Hershey's Unsweetened Cocoa can, and the second recipe is how my mother always made her Chocolate Frosting. Having grown up on my mother's, I prefer it but I have tried the Hershey's version as well. Both are good and either would be good with Thunder Cake. You could also use some strong black coffee instead of milk or water for a Mocha Frosting if you like.

Hershey's Chocolate Buttercream Frosting

6 Tbsp butter
3/4 cup cocoa
2 2/3 cups powdered sugar
1/3 cup milk
1 tsp vanilla

Cream butter in small mixer bowl. Add cocoa and powdered sugar alternately with milk; beat to spreading consistency (additional tablespoon milk may be needed) Blend in vanilla.

My Mom's Chocolate Buttercream Frosting

About 3/4 of a 1 lb. box of confectioner's (powdered) sugar
3 heaping Tbsp. unsweetened cocoa (or more if you want it more chocolatey)
4 or 5 Tbsp. butter, softened
2 to 3 Tbsp. water, or more as needed
Pinch of salt (just a pinch - it cuts the sweetness a bit)

Cream the butter, powdered sugar, salt and cocoa together with an electric mixer on the lowest power. Once the sugar and cocoa are incorporated you can raise the speed. (You don't want to be covered with powdered sugar and cocoa :-)! ) Add the water - start with 2 Tbsp., and mix, then add more water, a tsp. at a time, until frosting is of desired spreading consistency. Spread on cooled cake. For white frosting, simply omit the cocoa.