First, prepare the sour cherry layer. Chop the sour cherries. Add 100 ml of sour cherry juice, liqueur and sugar and bring to a boil. Whisk the re- maining juice and water with starch until well combined and add it to the cherries while stirring constantly. Cook until it thickens up to a pudding consistency. Cover a cake ring 14 cm in diameter (4 cm smaller than the diameter of the cake) with plastic wrap, bringing it up to the sides. Place the ring on a flat and hard surface, pour the sour cherry filling into it, level it out and put in the freezer to set.
Preheat the oven to 180 °C (350 °F). Beat the eggs and sugar at maximum speed for 10 minutes. Stir the dry ingredients and sift them on top of the egg mixture. Stir them gently with a spatula. Spread the batter evenly on the baking tray, 18 cm in diameter, lined with parchment paper (the bottom). Bake for 15 minutes. Take the baked sponge cake out of the baking tray and cool it on a cooling rack.
Warm up the milk and dissolve the cacao. Add cherry liqueur and cool. Melt the chocolate with 70 g whipping cream in a double boiler, whip the rest of the cream and refrigerate. Soak the gelatin in cold water for 5 mi- nutes, wring it out and dissolve it in hot chocolate. Cool the chocolate until lukewarm, then fold in the whipped cream with a spatula.
Cut the sponge cake into two equal layers. Place the bottom, bigger part of the sponge cake on a stand, moisten it and fit the ring. Coat it with just under half of the filling and level out the surface. Cut the top part of the sponge cake to a diameter of 14 cm (4 cm less than the diameter of the cake), place it on the filling and moisten; put the frozen sour cherry layer on top. Coat with the rest of the filling and level out the surface – be sure to fill the edge around the sponge cake first. Place the cake in the refrigerator to chill.
Once the filling has chilled slightly, glaze with ganache: bring the cream to a boil and pour it over the chocolate. Wait one minute and then mix with a spatula until well combined. Place the cake back in the refrigerator to set. Remove the rim from the chilled cake and decorate as desired.