Pour the berry mixture into a sieve and let it thaw. I did this the evening before so that the fruits don't give off so much moisture when baking.

Mix the butter, sugar and vanilla sugar into a smooth mixture - approx. 5 min. Now stir in the flour. When the mass is well connected - possibly knead the rest together by hand - remove approx. 300 g for the crumble and place in a cool place. Now put the egg in the rest of the dough and knead it underneath. Wrap in foil and put in the fridge for about 30 minutes.

Beat egg whites with salt until stiff and set aside. Mix the butter and sugar together until creamy, then stir in the egg yolks. Now the quark and the pudding powder come to the mass and are also mixed in well. Finally fold in the stiffly beaten egg white.

Line a springform pan with baking paper and lightly grease the edge. Take the dough for the base out of the refrigerator and press it into the mold - form a small edge. Prick the bottom several times with a fork.

Spread the quark mixture on the floor and pour the fruit mixture on top. Use the remaining dough for the crumble and knead in the oat flakes until it is nice and crumbly. Spread the crumble on the fruit and bake the cake at 175 ° C for about 55-60 minutes.

Let the cake cool completely in the tin and sprinkle with powdered sugar before serving.