How to make it:Separate eggs. Beat together the butter, 2/3 of the sugar, vanilla extract and egg yolks until creamy. Fold in the cream. Mix flour and baking powder together and sift over top of the liquid mixture. Beat the remaining sugar with the egg whites until stiff and mound it on top of the liquid mixture. Carefully fold everything together.
Divide the batter into two halves. Mix the cocoa together with a bit of water and carefully (to ensure that the egg whites are not "overbeaten") mix it into the one half of the batter. Alternating with dark and light, transfer the batter into a Gugelhupf mould or bundt pan that has been buttered and coated with bread crumbs. Finally, use the handle of a wooden spoon and zigzag through the batter several times to create the distinctive marble effect. Bake at 180°C (350°F) for about one hour.