Chop the dark chocolate and place it in a heatproof bowl.
Heat the heavy cream in a saucepan until it simmers but does not boil.
Pour the hot cream over the chopped chocolate and let it sit for a couple of minutes.
Stir the mixture until smooth, then add the orange zest and juice.
Add the icing sugar gradually and mix until fully incorporated.
Cover the bowl with plastic wrap and refrigerate until firm, about 2 hours.
Once firm, use a melon baller or small scoop to form balls from the ganache.
Roll each truffle in cocoa powder to coat.
Store the truffles in an airtight container until ready to serve.