Whip the cream until stiff. Mix mascarpone, sugar, vanillin sugar and orange liqueur, while pouring in the cream firming agent. Fold in cream and spread loosely on the fruit base. Chill the cake for about 20 minutes.
In the meantime peel oranges in such a way that the white skin is completely removed. Cut oranges into thin slices and cut them once to the middle. Cover cake with orange slices and decorate with pistachios.