150 g butter
250 g sugar
250 g flour
1 tbsp baking powder
4 eggs
1/2 cup milk
3 tbsp grated chocholate or cocoa powder

Mix well butter with sugar and egg yolks, then add gradually flour, milk and baking powder.
Beat egg whites until stiff and add to batter carefully.
Divide into 2 parts and add grated chocholate or cocoa powder into one part.
Grease the baking pan.
Fill the baking pan alternately with 1 tbsp of yellow and 1 tbsp of brown batter.
Even everything and bake at moderate temperature.
Cool and cut into rectangulars.
Optionally, sprinkle with icing sugar.