Separate eggs. Beat egg whites into snow and set aside. Mix the yolks and icing sugar until frothy. Stir in cocoa, sour cream and the grated hazelnuts. Finally stir in the flour sifted with baking powder and the egg whites. Spread the dough on a greased baking sheet.
Bake in the preheated oven at medium heat (180 degrees) for about 20 minutes.
Whip the whipped cream for the cream. Stir in the icing sugar and gradually the cream cheese. Round off with rum and vanilla sugar. Spread this cream on the cooled cake and top it with fruit of your choice (in winter, for example, I like tangerines with chocolate flakes on them).
Note: The recipe is designed for one baking sheet. If you take a round cake pan, the baking time (depending on the stove) should be around 40 - 45 minutes.