Mix the butter, sugar, vanilla sugar, egg yolks and rum until frothy. Gradually stir in the milk. Mix the flour and baking powder and fold in alternately with the stiffly beaten egg whites. Spread half of the mixture on a greased, floured tray.
Mix the ingredients for the filling and spread on the dough. Mix the second half of the dough with the cocoa powder and spread on the quark mixture.
Bake at 160 ° C for about 45 minutes. After cooling, sprinkle with powdered sugar.