For flambéed chestnuts, cut the chestnuts crosswise at the top. Place on a baking tray and bake in the preheated oven on the middle shelf for 20 minutes at 200° C (gas mark 4).
Peel and let cool.
Then put a layer of chestnuts in a small ovenproof dish. Spread half of the butter on top. Sprinkle with 2 tablespoons of sugar. Again put chestnuts and on top butter slices, remaining sugar and a pinch of salt.
At the table, pour cognac over them, light the fire and enjoy the flambéed chestnuts.