For the chocolate cake, preheat the oven to 180 °C top/bottom heat. Melt the chocolate in a water bath. Add a tablespoon of the butter.
Grease and flour the cake pan. Separate the eggs. Beat the snow with a pinch of salt until stiff. Cream butter together with icing sugar, vanilla sugar and lemon zest. Gradually stir in the yolks. Add melted chocolate. Mix flour and baking powder. Fold in the snow alternately with the flour and baking powder mixture.
Now pour the batter into the pan and bake on the middle rack for about 50 minutes. Place glaze in the water you used for the water bath earlier and melt. After about 40 minutes, place aluminum foil over the cake to prevent the surface from turning black. To be on the safe side, do a pin test before removing and if nothing sticks, remove from the oven.
Loosen the cake ring and transfer the cake to a baking sheet covered with aluminum foil. Spread the cake with the jam, then pour the chocolate icing over it and let the cake cool.