Put the cold butter, 200 gram of flour and the icing sugar together in a foodprocessor and mix for a few minutes until you have a firm dough. Turn out onto a kitchen counter and quickly knead until it all comes together. Wrap in clingfilm and leave in the fridge for at least 15 minutes. Roll it out a bit and press it into a greased caketin of 22 cm diameter. Make sure you have a standing edge of about 5 cm. Bake the bottom for about 15 minutes and then remove from the oven.
Bring a layer of water to the boil in a saucepan and put a heatproof bowl on top of the pan and melt 180 grams of dark chocolate together with the 130 grams of butter (au bain marie). Remove the bowl from the heat. Mix the egg yolks in one by one, add 1,5 tbsp of cacao powder, 50 gr of the sugar and 250 gr of chestnut puree. Mix it all together well.
In a clean bowl whip the eggwhites to stiff peaks. Add the other 50 grams of sugar bit by bit. Fold in the eggwhite in batches into the chocolate mix. Spoon the mix onto the cakebottom, place in the middle of the oven and bake for about 25 minutes or until done. Leave to cool on a cake rack.
Stir in a bowl the ingredients for the garnish and sprinkle with a little sieve on top