Line a baking tray (approx. 35 x 40 cm) with baking paper. Separate the eggs. Beat the egg whites, 6 tbsp. cold water and salt with the whisks of the mixer until stiff. Sprinkle 150 g sugar on top.
Stir in the egg yolks one by one. Mix flour and baking powder, sieve onto the egg mass and carefully fold in.
Spread the sponge mixture evenly on the tray.
Bake in a preheated oven (electric oven: 200 °C/circulating air: 175 °C/gas: see manufacturer) for 8-10 minutes. Take out and turn onto a damp tea towel. Remove the baking paper and place a second damp cloth on top.
Let the sponge cake cool down.
Drain mandarins well. Soak gelatine in cold water. Mix quark with 75 g sugar, 1 packet of vanilla sugar, lemon peel and lemon juice. Squeeze the gelatine. Dissolve at low heat.
Stir in 2-3 tablespoons of quark cream, then stir everything into the rest of the cream. Whip 400 g cream until stiff and fold in. Spread the quark cream on the sponge cake plate. Spread mandarins on top.
Cut the sponge cake lengthwise into 6 equally wide strips. Put the strips together spirally on a cake plate to form a cake. Place a cake ring firmly around the cake and chill for approx. 4 hours.
Whip 300 g cream with 1 packet of vanilla sugar until stiff. Loosen the cake ring and spread the cake all around with cream. Decorate with chocolate eggs and pistachios.