Grease the tin very well (really thick and all over) and flour it.
Weigh out the flour and mix with the baking powder in a separate bowl.
Separate the egg yolks from the egg whites.
Weigh out the butter and sugar and place in a bowl with the vanilla sugar and egg yolks.
Beat the egg whites until stiff.
Beat the butter, sugar, vanilla sugar and egg yolks until fluffy. (It takes about 5-10 minutes until the mixture becomes lighter in color).
Preheat the oven to 150°C (~300°F).
Gradually sift the flour into the foamy yolks and mix.
Fold in the beaten egg whites.
Pour half of the mixture into the bundt cake tin.
Mix the other half with sieved cocoa powder and also pour into the tin. Optionally, use a stick (skewer, chopstick, etc.) to draw very light circles through the batter to enhance the marbling. (My mother-in-law skips this step and the marbling still works).
Now bake in the oven for 1 hour. After the baking time, leave to cool completely in the tin. Then turn out with the help of a plate and serve sprinkled with powdered sugar.