Finely chop or break the dark chocolate into small pieces.

Pour the heavy cream into a saucepan.

Heat the cream over medium heat until it is just about to boil. Remove from heat immediately.

Add the chopped chocolate to the hot cream.

Let the chocolate sit for a minute to soften, then stir gently until completely melted and smooth.

Stir in the rum.

Place the bowl of chocolate mixture over a larger bowl filled with ice water.

Stir the chocolate mixture constantly until it begins to thicken and harden slightly.

Once the mixture is firm enough to hold its shape, use a tablespoon to scoop out portions of the truffle mixture.

Place the spoonfuls onto a baking tray lined with parchment paper or a silicone mat.

Place the tray of truffle mounds in the refrigerator and chill for 10 minutes to firm up.

Remove the chilled truffle mounds from the refrigerator.

Roll each mound between your palms to form smooth, round balls.

Place unsweetened cocoa powder in a shallow dish.

Roll each truffle ball in the cocoa powder, coating it completely.

Return the coated truffles to the baking tray.

Place the tray of truffles back in the refrigerator.

Chill for at least 2 hours, or until the truffles are firm.