This is the classical German marbled cake. There are several recipes for marbled cakes posted here on food.com but none is the authentic one, somewhat similar to a pound cake.
Preheat oven to 350 degrees and grease a bundt cake pan.
Beat butter and sugar together, then add vanilla, eggs and salt.
Combine flour and baking powder and stir into butter-sugar-egg mixture, alternating with the milk. Only add enough milk for the batter to drop heavily and ripping apart from a spoon.
Fill 2/3 of the batter into the greased cake pan.
Mix the remaining batter with cocoa, sugar and 2 or 3 tablespoons milk, until batter again drops heavily and ripping apart from a spoon.
Pour cocoa batter on top of the light one and move a fork spiralling through the two layers, so that you get a marbled pattern.