Step 1In the bowl of a food processor, finely chop the chocolate (it should be almost powdery). Alternatively, melt the chocolate in a medium bowl set over a pot of simmering water.
Step 2Meanwhile, in a medium saucepan, bring the cream to a simmer. Immediately remove from heat and combine with the chocolate: If using a food processor, pour the cream into the processor bowl while pulsing (the cream will melt the chocolate); alternatively, stir the cream together with the melted chocolate until the chocolate has melted. Add the vanilla.
Step 3Place the melted chocolate-cream mixture in a metal bowl set into an ice bath. Use a whisk to whip the mixture just until it thickens sufficiently to use as a frosting. Remove from the ice bath. Use immediately or store, covered, at room temperature for up to overnight.