Marbled Chocolate

ingredients:
1 egg
40g butter
20g sugar
2 tablespoons milk
65g flour
1/2 teaspoon baking
1 sachet of vanilla sugar
1 teaspoon cocoa powder
method :

Preheat oven to 180 ° C (gas mark 6).
Separate the whites from the yolks. In a bowl, mix the egg yolks, sugar and softened butter until the mixture becomes foamy.
Add the flour and baking powder and milk. Beat the egg whites until stiff, then gently incorporate the preceding mixture.
Divide dough into two equal parts: one, add the vanilla sugar. In the other, the cocoa powder.
In an individual cake pan, pour the two mixtures alternately.
Cook all about 30 minutes. 
http://cuisinesolo.blogspot.com/2010/10/marbre-au-chocolat.html