Put the dark and milk chocolate in a bowl, then put the cream and butter in a pan and bring to a simmer. Pour the hot cream over the chocolate and stir until it melts. Leave to cool, then chill in the fridge for 7 hrs.
Put the coatings into separate bowls. To shape the truffles, lightly rub your hands with flavourless oil and roll teaspoons of the truffle mix between your palms – this can get messy!
Gently roll the truffles in the bowl until evenly coated, then put in a box and chill. Store in the fridge in an airtight container for three days, or freeze for up to a month. Defrost in the fridge overnight.