Wash the orange thoroughly and finely grate the zest. Put the cream, butter and icing sugar in a saucepan and bring to the boil. Break the chocolate into small pieces and add to the cream mixture. Stir until the chocolate has melted. Stir in the orange zest.
Place in the refrigerator until the truffle mixture stiffened. Shape the mixture into balls, place them on a plate and put them in the freezer for 30 minutes.
Finely chop the pistachios. Melt the chocolate over a water bath. Dip the truffles in the chocolate, then roll them in the chopped pistachios. Stand them on a sheet of foil to set.
Store in a cool place.