pour over as a glaze, or whip it for an icing. If you don't like the sweetness level you could use milk chocolate for sweeter ganache, or bitter-sweet for less sweetness. Be sure to use fresh or good quality chocolate.
heat the cream until its simmering. Don't get frustrated while you're waiting and blast it up to high heat, medium to low heat is safer.
once its simmering (little bubbles are appearing), stir in the rum, and pour over chocolate stir
don't get discouraged, even though it looks like it can't mix at first, just keep stirring and it should eventually mix. At this point, you can dump it over a frosted cake for a smooth look, or wait for it to cool and whip it for a rich chocolately frosting.