Break the chocolate into a heatproof bowl and melt over hot, but not boiling, water, stirring occasionally. Remove from the heat and stir in the cream and Cognac or brandy. Sift in the icing sugar and mix until smooth. Place a sheet of cling film directly on the surface of the mixture and chill in the fridge for 1 hour, or until firm enough to handle. Mould into small, rough ovals (to resemble real truffles) and roll in the sifted cocoa. Pack into airtight containers and store in the refrigerator or freezer until serving.