Put the orange juice and zest in a small pan and simmer until the liquid has reduced to 1tsp.
Put the chocolate and cream in a bowl and melt in 20-second blasts in the microwave, or set over a pan of simmering water (make sure the base doesn't touch the water). Stir in the orange liquid. Pour into a container that will give the mixture a depth of at least 2cm (¾in) and leave to cool completely. Once cool, put in the fridge to chill for 2-3 hours, until the mixture is firm enough to shape.
Use a melon baller or teaspoon to scoop out and form balls. Roll each truffle in icing sugar (if using your hands dust them with icing sugar first). If there is any leftover mixture, amalgamate and chill, then shape the truffles as before.
Truffles are often made with fresh cream, which will reduce the amount of time they can be kept. Truffles can be frozen in an airtight container for up to a month. They can be kept in the fridge in an airtight container for up to a week. If you are not using fresh cream they could also be kept in a cool dark place for up to a week.
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