Finely chop or slice the chocolate. Whip the cream until stiff. Stir the chestnut puree with rum, vanilla sugar and possibly a little sugar until smooth. Add the mascarpone. Add chocolate and cream. Mix well and refrigerate the mousse for 2 hours. To serve, form balls with a glass spoon. Arrange on dessert plates and garnish with chocolate shavings, powdered sugar and marron glace.
Tip: Feel free to use better chocolate – the more delicious the result!