Drain the cherries on a sieve. Collect the juice and boil it in a pot. Mix the starch and 4-5 tablespoons of water. Add to the juice while stirring constantly and bring to the boil again briefly.
Fold in the cherries and allow to cool. Cut the cake into 4-5 cm wide slices and halve. Line the bottom of a bowl (3.5 litres capacity) with them. Fill any gaps with smaller cake pieces.
Whip 1 cup of cream until stiff. Add vanillin sugar and cream setting agent. Place the yoghurt in a bowl and fold in the cream. Cover the cake with the cream. Carefully spread the cold stewed cherries on the yoghurt mixture.
Whip the rest of the cream until half stiff and spread it on the cherries. Serve with chocolate shavings and, if desired, decorated with lemon balm.