Preheat oven to 165 degrees C. Butter a cake tin and dust with flour.
In a bowl, mix flour, baking powder and salt. Set aside.
In another bowl, with an electric beater beat the butter, sugar and vanilla sugar. Add the eggs one at a time and beat until smooth. Gradually beat in the flour mixture. Finally beat in the milk to make a fluffy batter.
Transfer 1/3 of the batter to another bowl and beat in the 2 tablespoons cocoa. Spoon the vanilla batter and chocolate batter alternately into the tin to creat a marble effect, finishing with a layer of vanilla batter.
Bake 55-60 minutes, or until it tests clean with a skewer. Let cool 5 minutes in the tin then cool on a wire rack.