For the marble cake, cream the butter, gradually adding the sugar and vanilla sugar.
Beat in the eggs, one at a time. Mix the flour with the cornstarch, baking powder and salt, stir into the meringue alternately with the milk.
Preheat the oven to 180 °C. Grease the baking pan well with butter.
Fill half of the batter into the mold. Mix the cocoa into the remaining batter. Pour the dark batter over the light batter.
Mix lightly with a fork to create the marble pattern. Bake the cake in the brook oven on the middle rack for 1 hour, testing with a wooden skewer.
Turn the slightly cooled cake out of the mold and sprinkle with powdered sugar.