Chocolate Cake: Preheat oven to 400 degrees Fahrenheit. Place the egg yolks in a bowl, Beat with sugar until thick. Mix hot water and melt chocolate, then combine with the egg yolk and sugar mixture. Beat the egg whites until stiff, then fold into the previous mixture. Pour the mixture into a slightly oiled baking dish. Using a spatula or spoon, push the mixture to the edges. (Find a dish that will let you have at least a ½-inch thick mixture when you fill it. Bake for 15 minutes. Let cool 10 minutes before removing from baking dish. Chestnut Filing: In a blender or food processor, mix chestnut with just enough milk for a thick puree. The mixture should be soft but not flowing about freely. Assemble: Use a cookie cutter to cut out a chocolate cake crust. Put some chestnut filling on the chocolate cake, then top with another piece of cake. Another way is to divide the chocolate cake crust into two halves. After spreading the chest filling on one half, top with the other. Then you can cut out any size and shape you like. (Optional) Top with your favorite jam or some fresh cut fruit.