Separate the eggs. Mix the lukewarm butter with the icing sugar, the vanilla sugar and the grated lemon zest until very frothy. Gradually add the egg yolks. Beat the egg whites with the salt and the raw sugar until stiff. Now carefully stir the flour and snow into the yolk mixture. Possibly loosen with a little milk. Color a third of the dough with melted chocolate. Butter and flour a bundt pan. Pour in half of the light mass, pour the chocolate mass on top and finish with the remaining light mass. Smooth the surface of the dough and bake the Gugelhupf in the slightly preheated oven on the middle rack at 140 ° C - 150 ° C with a fan for about 1 1/4 hours (normal stove 170 ° C). Do a needle test! Fall onto a work surface dusted with flour and allow to cool. Remove from the mold and sprinkle with icing sugar.