Cream butter, sugar and vanilla sugar, gradually fold in the eggs.
Sift flour with baking powder and add little by little. Depending on the consistency of the dough, add a little milk if necessary (the dough should fall from the spoon viscously).
Pour half of the dough into a greased marble cake pan.
Mix cocoa with milk and add to the second half of the dough.
The batter should be a bit more “runny” than the light batter.
Pour the dark batter over the light batter and stir in with a fork in a spiral pattern.
Bake at 160 °C hot air for about 65 minutes.
Allow to cool slightly in the pan, then turn out and sugar to taste.