Place the chocolate in a large metal or glass bowl. Heat the cream in a saucepan just to the boiling point. Pour the cream over the chocolate, making sure that all of the chocolate is covered. Cover the bowl and let stand for 5 to 10 minutes.
Whisk the chocolate until dark and shiny, then cool to room temperature. To thicken, beat cooled icing with a hand mixer for a few minutes. It's best if used after 12 hours.