Beat the butter with 1/2 the sugar. To this add the yolks 1 by 1, while stirring constantly.
Beat the egg whites with the rest of the sugar to snow. To the first mixture, add 1 tbsp egg whites, 1 tbsp flour, 1 tbsp yoghurt and so on until you run out.
Set aside 2-3 tbsp of the mixture to color with the cocoa.
Pour 1/2 the mixture into an oiled and floured form. To it add the cocoa-colored part and finally the remaining 1/2 of white mixture.
Bake in a moderate oven.