Soak the gelatin sheets in a bowl of cold water until softened. Reserve four whole walnut kernels for garnish. In a food processor, blend the remaining walnuts with the sugar until finely ground, then mix with the chestnut cream.
Melt the softened gelatin in a small amount of boiling water, then stir into the chestnut-walnut mixture.
Whip the cold liquid cream to soft peaks. Gently fold it into the chestnut mixture using a spatula to maintain lightness and volume.
Beat the egg white with a pinch of salt to stiff peaks, then carefully fold into the preparation.
Cut four strips of white card stock, 10 cm wide and slightly longer than your ramekins. Wrap each ramekin with a strip, allowing a 5 cm overhang, and secure with tape. Divide the mixture among the ramekins, filling nearly to the top of the collar.
Refrigerate for at least 4 hours until set. To serve, gently remove the card stock collars, dust with icing sugar, and garnish with reserved walnut kernels and fresh mint leaves.
