Make the Ganache:

1. In a medium heat-proof bowl set over a pan of simmering water, melt the white chocolate chips. Add the heavy cream, butter, dark rum and nutmeg, and whisk together until the mixture is thick and smooth.
2. Cover the bowl tightly with plastic wrap and chill until firm enough to scoop, about 1 hour.

Shape and Coat the Truffles:

1. Line a baking sheet with parchment paper, and dust your hands with a little icing sugar.
2. Scoop heaping teaspoons of truffle mixture and roll into balls, dusting your hands with more icing sugar as needed. If you find the ganache is getting too soft to roll easily, transfer it to the freezer for 5-10 minutes to firm up again.
3. Using a fork, dip the truffles into the melted white chocolate, and place on the prepared baking sheet. (Or, if you prefer a more low-maintenance option, roll the truffles in a thin layer of ground almonds instead.)
4. Place the finished truffles in the freezer for 10-15 minutes to firm up, then transfer to an airtight container and refrigerate for up to 1 month.