Melt the dark chocolate and butter together in the microwave on medium heat for about 4 minutes stirring a couple of times during cooking. Stir in the mixed peel, Cointreau, orange zest and icing sugar until well combined. Chill in the fridge until firm enough to handle and roll into balls about the size of a walnut.
Place the balls on a foil-lined tray and freeze until really solid about 2 hours. Melt the white chocolate in the microwave on medium power or over a pot of hot water, dip the truffles in the melted chocolate. A chocolate dipping fork which are available at
good cookware shops is invaluable for this job. Allow excess chocolate to drip off and leave to set on a foil-covered tray. DO NOT store in the fridge as the chocolate tends to sweat, but keep them in a cool place. A good idea is to keep a plastic container of un-dipped truffles in the freezer, to be removed and dipped as you need them, for after dinner treats or small gifts.
Special Instructions: For cappuccino truffles replace Cointreau with coffee liqueur or essence and delete the mixed peel and grated orange rind. These can be dipped in white or dark chocolate.
For peppermint truffles delete Cointreau and mixed peel and orange rind, for 1-2 teaspoons of peppermint essence.
For toasted coconut and liqueur truffles, replace the Cointreua, mixed peel and orange rind with Irish cream liqueur or bailies etc. and add 1/2 cup toasted coconut.