Preheat the oven to 180°C (approximately 350°F). Butter a 22 cm (approximately 9-inch) bundt cake pan and dust with breadcrumbs.
In the bowl of an electric mixer, beat the butter with the sugar and the vanilla sugar until creamy. Beat in 1 pinch of salt, and the lemon zest. Beat in the eggs one at a time, beating well after each addition. Set aside 5 tablespoons of the milk. In a bowl, stir together the flour, cornstarch and baking powder. Alternately beat in the flour mixture and the remaining milk to the butter mixture, beginning and ending with the flour mixture.
Divide the batter in half and add the cocoa powder and the remaining milk to one half. Alternately add the chocolate batter and the plain batter to the pan and pull a long fork through the batter to create a marbled effect.
Bake until a wooden pick inserted near the center of the cake comes out clean, about 1 hour. Cool the cake in the pan for 15 minutes, then run a metal spatula around the edge and the center and invert the cake onto a wire rack to cool completely. Dust with powdered sugar before serving.