Preheat the oven to 350°F / 180°C. Grease and flour a Bundt cake pan.

Using an electric mixer, beat the butter, sugar and eggs together until creamy; about 5 minutes. Stir in the vanilla sugar and a pinch of salt.

In a small mixing bowl, measure out the flour, cornstarch, vanilla powder, and baking powder; whisk together to combine.

Add the flour and cold milk to the creamed eggs, alternating. Add 1/3 of the flour to the batter and beat on low speed just until incorporated. Then add 1/2 of the milk and beat. Add another 1/3 of the flour, and beat, and then the last of the milk and beat. Finally, beat in the last 1/3 of the flour. Add a little more flour if the dough is too runny.

Fill 2/3 of the amount of batter into the well-greased cake pan.

Add the cocoa and additional sugar to the remaining batter along with 2-3 tablespoons milk. Mix together until smooth.

Pour the remaining batter into the cake pan. Now with a fork (or handle of a wooden spoon), swirl the batter to create a marbled effect. This is done by moving the fork in a spiraling pattern through the cake, mixing the two dough layers, resulting in a marble pattern in the dough. Hence the name of the marble cake.

Place in the oven and bake on the lower third of the oven for about 60-75 minutes, rotating the pan half way through. The cake is done when it has browned slightly and a cake tester (or toothpick) comes out clean when inserted into the middle.

Allow to cool on a rack for 10 minutes. Then, gently loosen the edge of the cake with a knife from the baking pan. Give the marble cake a couple of gentle shakes, so that it releases itself from the form. The cake will come out clean and whole without sticking. Turn the cake upside down onto a wire rack and let cool completely in the pan, about 2 hours.

TIP: After the marble cake has completely cooled, either dust it with powdered sugar, or cover with a chocolate glaze and serve. This classic sponge cake tastes best when cut and served one day after baking.