Preheat the oven to 180°C (350°F). Line a 20 cm cake pan with parchment paper.
In a large bowl, beat the eggs and sugar with a mixer until pale, thick, and creamy.
In a separate bowl, sift together the flour, cocoa powder, and baking powder.
Gently fold the dry ingredients into the egg mixture. Add the vanilla sugar or vanilla extract and milk, then mix until smooth.
Pour the batter into the prepared pan and bake for 20 to 30 minutes, or until the cake springs back when lightly touched.
Let the cake cool completely, then slice it horizontally into 3 even layers.
To make the cherry filling, place the cherries and sugar in a saucepan and cook over low heat until the fruit softens and releases its juices.
In a small bowl, mix the cornstarch with the water. Stir it into the cherries and cook for about 1 minute, until the filling thickens. Let it cool completely.
To make the mascarpone cream, whip the heavy cream until soft peaks form. Add the mascarpone and powdered sugar, then beat until thick, smooth, and stable.
Divide the cream into 3 portions, reserving a little extra for frosting the outside of the cake.
Place the first cake layer on a serving plate or in the pan. Lightly brush it with a little milk, then spread with a layer of mascarpone cream.
Add the second cake layer. Brush lightly with milk, spread with more mascarpone cream, and top with the cooled cherry filling.
Place the third cake layer on top. Lightly brush with milk and cover the top with the remaining cream.
Refrigerate the cake for at least 3 hours to set.
Frost the sides and top with the reserved mascarpone cream.
Decorate with melted chocolate, chocolate shavings or curls, and candied cherries.
Slice and serve.