Cream butter with sugar and vanilla sugar, and gradually stir in 1 pinch of salt, lemon zest, and eggs.
Mix the flour, cornstarch, and baking powder in a bowl and add slowly to the egg mixture, alternating with the milk. Hold aside 5 tablespoons of the milk.
Divide the dough in half and add the cocoa powder and the remaining milk to one half. Grease a bundt cake pan with butter and coat with breadcrumbs. Pour the light and dark batter alternatingly into the pan. Pull a knife through the batter to marble the cake.
Bake the cake in a preheated oven at 180°C (approximately 350°F) on the middle shelf for about 1 hour. Allow the finished cake to cool in the pan, then turn out on a cooling rack. Dust the cooled cake with powdered sugar and serve.