1Place chocolate in a large heatproof bowl. Combine cream and butter in a small saucepan. Stir over medium heat until almost boiling.
2Remove from heat and pour over chocolate. Set aside 2 mins, then stir until smooth. Mix in rum. Cover with plastic wrap and chill overnight or until firm.
3Line an oven tray with baking paper. Place cocoa in a shallow bowl.
4Using a melon baller or a spoon, scoop teaspoons of mixture and shape into balls using the palms of your hands. Toss in cocoa to coat. Arrange on tray and chill until ready to serve.