Marmorkuchen

This is the other of the two cakes that my Dad likes the best, of the many things that I bake, he has always maintained that it tastes exactly like his mother made it, which is quite an accomplishment, seeing as he grew up in East Africa, in what was Tanganyika at the time, a German protectorate, it's Tanzania now. I use modern ingredients now, I imagine that vanilla sugar and Rum flavouring were hard to come by there. This recipe is from an old Dr. Oetker recipe book.

Marmorkuchen
250 g Butter
250 g Sugar
3 tsp Vanilla Sugar
4 eggs
1 tsp Rum extract
some salt
500 g flour
2 tsp baking powder
approx. 1/8 L of milk
30 g cocoa
25 g sugar
2-3 Tbsp. milk

Stir the butter until it is smooth, then alternately add the sugar, vanilla sugar and eggs. Add the rum extract and salt last. Mix the baking powder with the flour, then add it through a sieve, alternately with the 1/8 L of milk (only add enough milk so that the dough falls stiffly from the spoon). Transfer about 2/3 of the dough to a greased Springform pan. Add the cocoa and sugar, again sieved, to the remainder of the dough. Add enough milk to make it of a consistency that it falls stiffly from the spoon. Add the chocolate dough on top of the yellow dough, then draw a fork through the underside dough, twisting the dough in a spiral so that some of the yellow portion comes to the top. Continue this technique all around the Springform pan. Bake at 350C for 75 minutes, the last 10 minutes with the oven off.