Beat the eggs, 125 g sugar and salt with the whisk of the hand mixer until thick and creamy. Mix the flour, starch and baking powder and stir into the egg foam alternately with the milk. Pour onto a baking tray lined with baking paper and smooth down. Bake in a preheated oven (electric cooker: 225 °C/ convection oven: 175 °C/ gas: level 4) for 7-8 minutes. Then immediately turn the sponge onto a damp tea towel sprinkled with sugar.
For the filling, soak gelatine in cold water. Drain the mandarin oranges. Finely dice the chocolate. Mix quark, 100 g sugar, orange juice and butter-vanilla flavouring. Whip cream until stiff. Squeeze the gelatine, dissolve lukewarm and stir into the quark cream. Cool until the cream begins to gel. Fold in mandarins and cream. Spread on the sponge cake plate, sprinkle with diced chocolate and roll up from the long side. Put in a cool place for about 3 hours.
Squeeze the gelatine, dissolve lukewarm and stir into the quark cream. Cool until the cream begins to gel. Fold in mandarins and cream. Spread on the sponge cake plate, sprinkle with diced chocolate and roll up from the long side. Put in a cool place for about 3 hours. Just before serving, dust with icing sugar and cut into pieces. Serve decorated with mandarins and white chocolate rolls