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Bring the cream and butter to the boil in a small pan, and put the milk chocolate in a heatproof bowl. Pour the hot cream mix over the chocolate, leave for 1 min, then stir until smooth and combined. Whisk in the rum, then chill in the freezer for 5-10 mins, or until just firm enough to roll.
Roll tablespoons of the mixture into balls in your hands, then transfer to a non-stick baking tray (you may want to wear gloves). Chill in the fridge for 1 hr, or until set. Meanwhile, melt the dark chocolate in a small heatproof bowl set over a pan of simmering water, and put the cocoa powder on a plate. Dip the chilled truffles into the melted chocolate using a fork, then shake off the excess and roll in the cocoa. If the cocoa starts to get too sticky, add another 1 tbsp.
Put the truffles on a tray or plate lined with baking parchment and chill for 1 hr to set, then package. Will keep in the fridge for up to one week.