Preheat the oven to 180°C (350°F). Grease and line a 23 cm (9 in) springform pan or prepare two pans for easier layering.
Make the chocolate sponge: in a large bowl beat the Eggs with the Granulated Sugar and Vanilla Sugar until the mixture is pale, thick and ribbon-like when the whisk is lifted.
Sift together the All-Purpose Flour, Cornstarch, Unsweetened Cocoa Powder, Baking Powder and Salt, then gently fold the dry mixture into the egg mixture in two additions, keeping as much air in the batter as possible.
Pour the batter into the prepared pan, smooth the top and bake until a skewer comes out with a few moist crumbs (about 25–35 minutes depending on pan size). Let the cake cool in the pan for 10 minutes, then transfer to a rack to cool completely.
While the cake cools, prepare the cherry compote: combine the Sour Cherries, Cherry Juice and a few tablespoons of the granulated sugar in a saucepan and bring to a gentle simmer.
Mix a small amount of the Water with an extra tablespoon of Cornstarch to make a slurry, then stir it into the simmering cherries and cook until the compote thickens. Remove from the heat and stir in a splash of Kirsch; allow to cool to room temperature.
Make a kirsch syrup for brushing: warm a little Water with a couple of tablespoons of the Granulated Sugar until the sugar dissolves, remove from the heat and stir in another splash of Kirsch. Let cool.
Whip the cream: chill your bowl, then whip the Heavy Cream with the Powdered Sugar, a pinch of the Vanilla Sugar and the Cream Stabilizer until it holds firm peaks.
Assemble the cake: slice the cooled sponge horizontally into three even layers. Place the bottom layer on a serving plate and brush generously with the kirsch syrup.
Spread a layer of the cooled cherry compote over the soaked base, top with a thick layer of whipped cream, then repeat with the second layer. Brush the final top layer with syrup, place it on the cake and cover the top and sides with the remaining whipped cream.
Finish and chill: shave the Dark Chocolate over the top and sides for a dramatic look and decorate with additional sour cherries. Chill the assembled cake for at least 2–4 hours (preferably overnight) to let the flavors meld and the filling set before serving.
