Combine cocoa together with instant coffee. Stir in warm water; mix to dissolve, until smooth.
Whisk egg yolks with sugar until it's smooth. Add vegetable oil to the mix.
Gradually stir in a mixture of cocoa-coffee.
Combine flour together with baking powder and soda. In thirds, mix into a bowl with the dough. Stir the flour into the dough, until it’s completely mixed.
To the bowl of whites, add salt. Beat until whites look like foam, and if you were to flip the bowl upside-down, they would not fall out.
Very carefully, mix in - beaten egg whites into the batter. Especially, carefully mix first portion of the whites, because whites and the dough have very different textures, which gradually will become one. Mix whites into dough by folding movements, top - down.
Pour the batter into the pan and bake in a preheated 300F oven, for about 50 minutes. Use a wooden toothpick to check if the dough is baked.
In the process of baking, especially in the first 30 minutes, do not open the oven door! Otherwise, there is a high probability that the cake will settle.
Remove the cake from the oven. Let the cake cool to a room temperature, then slice it into 4 layers. Soak the cake with sweetened whipping cream.
Beat the egg yolks, add water and condensed milk. Stir until smooth.
Cook all ingredients on a very low heat. Continue mixing the cream the entire time it is on the heat. Make sure nothing is getting stuck on the corners. During this processes, keep turning the heat up by a little bit. Make sure not to bring your mixture to boil. Keep on mixing and cooking, until mixture has the texture of condensed milk.
Remove the mixture form the heat and while it’s hot, add chocolate to it. Give it a few minutes to melt and mix it through. Let the mixture cool off to a room temperature.
In a separate bowl, mix butter; add the cream mixture, a few spoons at a time, until it's mixed well.
Putting Prague Cake Together
Now you are ready to put your cake together. Put one layer of cake at a time and top it off with cream. Finish decorating as you wish or you can move to the next step.
This step is optional: Bring whipping cream to a boil, add chocolate chips to it. Stir until the chocolate is all melted and smooth, let it cool to a room temperature. Drizzle the cake with chocolate ganache and sprinkle some grated chocolates on top.