For the best results, use high-quality Kirschwasser and fresh sour cherries. It's important to chill the cake thoroughly so that the flavors meld and the cream sets properly. This recipe assumes you have a pre-baked chocolate sponge cake (24-26 cm diameter), cut into three layers.
In a saucepan, combine 400g of the cherries with the sugar and heat until the sugar dissolves and the cherries release their juice. In a separate bowl, mix the starch with water (or cherry juice) until smooth.
Pour the starch mixture into the saucepan with the cherries, bring to a boil while stirring constantly, and cook until the filling thickens. Remove from the heat and stir in 2 tablespoons of Kirschwasser. Let the filling cool to room temperature.
Whip the heavy cream with the vanilla sugar until stiff. Slice the cake sponge into three even layers.
Place the first layer of cake on a serving plate. Evenly brush it with one-third of the Kirschwasser. Spread half of the cherry filling over it, and then pipe a quarter of the whipped cream on top.
Cover with the second cake layer. Repeat the process: brush it with another third of the Kirschwasser, spread the remaining cherry filling, and top with another quarter of the whipped cream.
Place the third cake layer on top. Lightly brush it with the remaining Kirschwasser. Cover the sides and top of the cake with the remaining whipped cream.
Transfer the remaining whipped cream to a piping bag and pipe 12 rosettes on top of the cake. Place a remaining cherry in the center of each rosette.
Sprinkle the sides of the cake with the chocolate shavings. Place the entire cake in the refrigerator to set for at least 2-3 hours, or ideally, overnight.
Before serving, let the cake sit for a short time at room temperature to allow the flavors to develop fully.