Stir pudding powder into 3-4 tablespoons of cold milk and 2 tablespoons of sugar until smooth. Boil up the rest of the milk and cream with the vanilla sugar. Add the pudding powder to the boiling liquid and let it boil for about 1 minute while stirring.
Rinse 4 moulds (150 ml capacity) with cold water and fill in the pudding. Put them in a cold place. Remove the seeds from the melons. Use a cookie cutter to cut out balls from the pulp. Cut the rest of the fruit flesh from the peel and puree separately.
Season purees with remaining sugar and lime juice. Thicken each with 1/2 tablespoon of food binder. Arrange the sauce on the plates. Turn the flan over and spread melon balls on top. Decorate with lime slices, zests and lemon balm as desired.