Place the chocolate into a medium bowl. Heat the cream in a small saucepan over medium heat. Bring just to a boil, watching carefully to ensure it doesn't overflow. When the cream has come to a boil, pour over the chopped chocolate and whisk until smooth. Stir in rum, if desired.
Allow the ganache to cool slightly before pouring over a cake. Start at the centre of the cake and work outward. For a fluffy frosting or chocolate filling, allow it to cool until thick, then whip with a whisk until light and fluffy.