Preheat the oven for the dough (electric oven: 175 °C/circulating air: 150 °C/gas: see manufacturer). Roughly chop the chocolate. Cream eggs, 200 g sugar, 1 pinch of salt and vanilla sugar with the whisk of the mixer. Mix 325 g flour and baking powder, stir in alternately with mineral water and oil. Fold in chocolate.
Pour the dough into a greased and flour-sprinkled square springform pan (24 x 24 cm; alternatively round, 26 cm Ø). Bake in a hot oven for about 35 minutes.
Drain the cherries and collect the juice. Fill up the juice with water to 600 ml. Select the berries and wash them carefully. Stir 50 g sugar, red fruit jelly powder and 6-7 tablespoons juice until smooth. Bring the rest of the juice to the boil, stir in the red fruit jelly powder, bring to the boil while stirring and simmer for about 1 minute. Remove from heat, fold in the fruit. Let cool off while stirring occasionally.
Let the cake cool down. Remove from the mould and place on a plate. Spread the groats on top. Mix milk and vanilla sauce powder well with the whisk of the mixer. Serve the cake with vanilla sauce.