Prepare all ingredients:
Cook the chestnuts 20 minutes in boiling salted water. When cooked, drain them and peel them.
Pass them through the food mill.
Blanch the softened butter along with caster sugar.
Add the melted dark chocolate,...
...chestnut purée,...
... candied chestnuts broken in pieces,...
...1 teaspoon of vanilla extract and 2 tablespoons of Cognac. Mix.
Pour this preparation into a square mold that is lined with plastic wrap.
Leave to harden in the fridge for 12 hours minimum.
Unmold...
...and decorate surface (cocoa, icing sugar...).