How to make a perfect Gugelhupf
1. Preheat your oven to 340°F /170 °C and coat the bundt pan with melted butter.
2. Whsik the butter with the egg yolk until foamy, add confectioner's sugar and stir until creamy. Add vanilla sugar and some lemon zest.
3. Beat the egg whites and the white sugar until stiff. Sift together the flour and baking powder. Mix half with the milk and the egg yolk, then fold in the egg whites and the rest of the flour, alternating between.
4. Pour less than half of this mixture into the pan. Add coca and rum to the rest and mix. Pour the darker batter on top of the lighter one into the pan.
5. Drag the handle of a wooden spoon through the batter in a wave-like motion to achieve the marble texture.
6. Bake for 50 – 55 minutes. Let cool for a few minutes, then take the cake out of the pan. Dust with confectioner's sugar.