Mix the curd with the whisk of the hand mixer until creamy. Separate the eggs. Stir egg yolks, milk, vanilla sugar, lemon juice and 50 g sugar into the quark. Stir in starch.
Whisk the egg whites and salt with the whisk of the hand mixer until stiff, adding 25 g sugar. Fold the beaten egg whites into the curd mixture. Pour the mixture into a greased springform pan (20 cm Ø).
Bake in the preheated oven (electric cooker: 150 °C/ convection oven: 125 °C/ gas: see manufacturer) for approx. 25 minutes. Reduce the temperature (electric oven: 125 °C/ convection oven: 100 °C/ gas: see manufacturer) and bake for a further 30 minutes, remove from the oven and allow to cool on a cake rack for about 1 hour.
Carefully remove the cake from the tin, place it on a cake plate and chill for another 2 hours.
Raspberries selected. Wash and clean the rhubarb, cut into pieces and mix with 50 g sugar. Leave to steep for 20-30 minutes. If necessary, add 2 tablespoons of water and cook covered to a compote. Pour into a bowl and let cool.
Stir in the raspberries. Spread half of the compote in the middle of the cake, add the rest of the compote.