Place the biscuits in a small bowl, add the kirsch and leave to infuse.
Place the water and chocolate in a small thick pan and dissolve over a low heat. Stir well until quite smooth, then remove from the heat.
Cut the butter into small flakes and stir into the chocolate until completely dissolved.
Add the egg yolks and beat until the mixture is smooth again.
Stir in the sugar, blend thoroughly, add the stiffly whipped egg whites and beat them in until mixed thoroughly.
Add the soaked biscuits and grapes.
Finally add the cream.
Turn into a 13cm/5in diameter, oiled sliding-based cake tin, being careful to line the base with a fitting circle of oiled greaseproof paper.
Refrigerate until set, turn out, remove paper and garnish.