Separate eggs. Beat the egg whites until stiff, adding 1 packet of vanilla sugar, 100 g sugar and salt. Add the egg yolks and stir in. Mix flour, starch and baking powder and sieve portions onto the egg mixture and fold in. Line the bottom of a springform pan (24 cm Ø) with baking paper. Fill in sponge cake mixture and smooth it down.
Bake in the preheated oven (electric: 175 °C/ gas: level 2) for about 25 minutes. In the meantime, whip cream until stiff with the whisks of the hand mixer. Mix mascarpone, quark, remaining vanilla sugar, remaining sugar, lemon juice and vanilla pulp. Fold in the cream in portions. Put the cream in a cool place. Remove the base from the oven, remove from the edge of the springform pan and allow to cool in the pan. Slice the base twice, put them back on top of each other and cut into 8 cake pieces. Remove the seeds from the yellow melon and cut in half again. Cut the flesh from the skin and slice. Cut the watermelon flesh from the skin and cut into slices. Cut melon slices into cake pieces.
Remove the base from the oven, remove from the edge of the springform pan and allow to cool in the pan. Slice the base twice, put them back on top of each other and cut into 8 cake pieces. Remove the seeds from the yellow melon and cut in half again. Cut the flesh from the skin and slice. Cut the watermelon flesh from the skin and cut into slices. Cut melon slices into cake pieces. Cover the cake pieces individually, spreading each sponge piece with cream first, then covering with melon. Dust the cake pieces with icing sugar and decorate with melon balls and lemon balm as desired.