Halve 100 g of biscuits. Mix cream, eggs, 150 g sugar, vanillin sugar, salt and oil with the whisk of the hand mixer. Mix flour and baking powder and stir in portions.
Fold in the biscuits. Put them into a greased springform pan (26 cm Ø) sprinkled with breadcrumbs and smooth them down. Bake in the preheated oven (electric cooker: 175 °C/ convection oven: 150 °C/ gas: level 2) for 25-30 minutes.
Remove from the oven, remove the base from the edge of the springform pan and allow to cool in the pan. Peel oranges and grapefruits so that the white skin is completely removed. Cut 1 orange and 1 grapefruit into slices.
Cut out the fillets from the others between the dividing skins. Collect the juice while cutting. Cover the fillets and put them in a cool place. Soak gelatine in cold water. Mix mascarpone, curd cheese, 125 g sugar and the collected juice.
Squeeze and dissolve the gelatine. Stir 2 tablespoons of cream into the gelatine. Stir everything into the remaining cream. Remove the cake base from the mould and enclose with a cake ring. Spread about 1/4 of the cream on the base.
Spread orange and grapefruit slices on top. Add the rest of the cream and spread loosely. Chill for about 2 hours. Remove the cake from the ring. Decorate with set aside fillets and 50 g biscuits.