I got the receipe below from a old Good Food of mine but have to share it as its so yummmy and not hard to make!!!
But I would make it the day before you need it if you want it to set a bit more in the fridge...
200g dark chocolate
200g unsalted butter
200g peeled cooked chestnuts
100 ml milk
100g sugar
4 eggs
Melt the chocolate and butter together in a pan over a very gentle heat. In another pan, heat the chestnuts with the milk until just boiling, then mash thoroughly with a potato masher (or process until smooth in a machine).
Separate the eggs and mix the yolks with the sugar, then mix together with the chocolate mixture and the chestnut mixture until you have a smooth, blended batter.Whisk the egg whites until stiff and fold carefully into the batter. Pile into a greased, lined, cake tin (the spring form type is good) and bake in a pre-heated oven at 180C for about half an hour.If you want to serve it warm, leave to cool for a minute or two, then release the tin, and slice carefully ? it will be very soft and moussey. Or leave to go cold, when it will have set firm.