In a small saucepan heat the heavy cream and rum until warm to touch, but not boiling. Remove from heat.

Stir in chopped dark chocolate and butter, and stir until smooth. Stir in brown sugar and whisk until combined.

Transfer to a bowl and allow to cool. Cover with plastic wrap and chill for at least 4 hours, preferably overnight. 

Once cool and solid, using a cookie scoop or tablespoon, portion the ganache into 1oz portions. Roll the balls between your hands and place on a baking sheet lined with parchment or wax paper. This process can get a little messy. 

Next, pour the cacao powder into a small bowl and dip and roll the truffles in the coating.

These truffles keep best in the refrigerator. Remove from fridge until ready to serve.