Line a 33x23cm Swiss roll tin with a double layer of baking parchment.
Preheat the oven to 200°C.
Melt chocolate and coffee with 85ml of water in a saucepan over low heat.
Beat egg yolks and caster sugar until pale and mousse-like before adding melted chocolate.
Whisk egg whites until stiff and gently fold them into the chocolate mixture.
Spread the mixture in the tin and bake for 12 minutes until firm.
Transfer the cake on its parchment to a wire rack to cool completely.
Whip the double cream and fold in the sweetened chestnut purée.
Turn the cooled sponge onto greaseproof paper and remove the lining parchment.
Spread the filling over the cake and roll it up tightly from the narrow end.
Wrap the log in paper and refrigerate for at least 30 minutes before serving.
Dust with icing sugar and garnish with holly before serving.