To prepare the chestnut mousse, soak the gelatin in a bowl of cold water until softened. Place the crème de marrons in a mixing bowl. Whisk until smooth, then mix in the whisky.
Heat 2 tbsp (30 ml) of the cream in a heatproof bowl over a saucepan of barely simmering water (bain-marie). Squeeze the gelatin to remove excess water, then microw