Line a baking tray (approx. 35 x 40 cm) with baking paper
Separate eggs. Beat the egg whites, salt and 2 tablespoons of cold water until stiff, adding 125 g sugar and vanillin sugar. Beat the egg yolks separately into the mixture. Sift flour, starch and baking powder into the mixture and fold in. Spread on the baking tray. Bake in the preheated oven (electric cooker: 200 °C/circulating air: 175 °C/gas: level 3) for approx. 12 minutes. Turn out onto a damp tea towel. Remove baking paper immediately. Let the sponge cake cool down
Drain the mandarins well and collect the juice. Mix quark, 5 tbsp. mandarin juice and 100 g sugar. Whip the cream until stiff, pour in the cream firming agent
Lift under the curd
Cut the biscuit 1 x crosswise to make 2 smaller plates. Sprinkle both plates with 1-2 tbsp. mandarin juice each. Spread 1/4 curd cream on a sponge cake plate. Cover with mandarins, up to approx. 15 pieces. Spread 3/4 of the remaining cream on top. Place the 2nd plate on top and press down a little. Spread the rest of the cream on it and pull it through with a cake comb or fork if necessary. Chill for about 1 hour.
Cut into about 15 pieces. Decorate with pistachios, lemon balm and other mandarins