Drain the mandarins and collect the juice. Halve the mandarins.
Fill up the juice to 1/4 l with water. Add the cornstarch, sugar and vanilla sugar and bring everything to the boil while stirring. Let cool while stirring (this works best if you put the pot in cold water).
Whip the cream until stiff and stir into the cooled cream. Fold in the mandarins and transfer to a glass bowl.