Melt the butter. Finely crush the lady fingers and mix with melted butter. Place a springform pan rim (24 cm Ø) on a cake plate. Put the sponge mixture into the mould and press it to a flat base.
Cold. Mix the jelly in a pot with 1/4 litre cold water and let it swell for 10 minutes. Stir cream cheese, 75 g sugar, vanillin sugar and lemon juice until smooth. Add remaining sugar to the jelly and heat while stirring (do not boil!).
Let it cool down. Whip the cream until stiff. When the jelly begins to gel, first stir in the cream cheese, then the cream (except for 2 tablespoons). Fill the mixture into the springform pan and smooth it down.
Pour the rest of the cream in small tuffs onto the cream and pull through loosely with a fork. Chill the cake for at least 2 hours. In the meantime cut out balls from the watermelon. Peel the Charentais melon wedges, remove the seeds with a spoon and cut the flesh into thin slices.
Remove the cake from the springform pan rim and cover with melon. Decorate with mint. Makes about 12 pieces.