Melt the chocolate, cream and butter in a heatproof bowl over a pan of simmering water, do not stir. Remove and stir gently, add nuts and leave to cool. Cover with clingfilm and refrigerate for 8 hours or overnight. Line a baking tray. Using teaspoon size of mixture, roll into approx 28 balls and place on baking tray. Return to refrigerator for a further 2 hours. Before serving roll the Pistachio Chocolate Truffles in icing sugar.