Melted chocolate with the butter in the bowl of water bath and keep warm. In a separate saucepan heat cream and when its temperature reaches that of chocolate - mix. Add the cognac and stir gently. Put it in the refrigerator until povtvardi, stirring occasionally. When the mixture is firm enough to put in a syringe with 2 cm nozzle make long strips on a baking dish coated with cooking paper. Put it in the refrigerator until hardened well, then with a sharp knife cut into pieces 5 cm long. Just before presenting the truffles are rolled in cocoa powder.