Melt the couverture and butter together in a water bath. Separate the egg whites from the yolks and heat the oven to 347°F (175 °C). Beat the egg whites to snow, and the egg yolks with the sugar until white. Sift the flour and baking powder. Carefully add the yolks gradually while beating continuously to have them cool.
Mix the egg whites in carefully, then begin to add the flour and mix everything well. Pour the dough for the layers into a cake form with removable walls and put it to bake for about 45 minutes in a moderate oven. Take the finished layer out of the form and leave it to cool. Carefully divide it into 3 separate layers. To prepare the filling, dissolve the starch in 3-4 tablespoons of juice.
Put the remaining juice to boil, take it off the stove, beat it with the starch and put it to boil once again. Immediately add the cherries, let boil for 1 minute and remove from the heat. Once the sauce cools, add half of the cherry liqueur. Use the remaining liqueur to soak the layers lightly.
Distribute the cherry mixture on 2 of the layers and let it cool in order to gel. In the meantime, beat the cream with the vanilla sugar. Carefully put the first layer with cherry mixture in a tray, smear it with 1/4 of the cream, place the second layer, then another 1/4 of the cream.
Place the third layer on and brush it and the entire Black Forest cake thoroughly with another 1/4 of the cream. Use the remaining 1/4 of cream to form roses along the edges of the cake with a pastry bag. Place a candied cherry on each rosette.
In the middle of the cake and along the sides, grate dark chocolate and allow it to cool for 2-3 hours.