Whip the butter and sugar foamy. Add the egg yolks while mixing. Add milk and flour to the mixture and combine everything together with a blender. Beat the egg whites into stiff peaks and gradually add them to the remaining mixture. Stir it with a spatula or blender at the lowest speed setting.
Divide the mixture into two parts. Add cocoa powder to one part.
Using a spoon, spoon out one mixture and then the other - a spoonful of white mixture and then a spoonful of dark mixture. Bake for 35-40 minutes at 180 °C.