Mix the cream with the porcini mushrooms, cover and chill for approx. 2 hrs. Strain the cream, set aside the mushrooms. Beat the cream until stiff.
Place the chocolate and cream in a thin-sided bowl and suspend over a gently simmering bain-marie, ensuring that the bowl does not touch the water. Melt the chocolate and stir until smooth. Combine the chestnut puree and kirsch. Stir in 2 tbsp of the chocolate sauce, carefully fold in the whipped cream. Transfer the mixture to a piping bag with a smooth nozzle (approx. 15 mm in diameter). Pipe the mousse into the glasses.
Mix the sugar with the reserved porcini mushrooms, caramelize in a frying pan while stirring. Top the mousse with the grapes, meringue and mushrooms. Drizzle the remainder of the chocolate sauce over the top.