1 cup heavy cream
8 ounces semisweet chocolate, chopped
Bring the cream to barely a simmer in a small saucepan and add the chopped chocolate. Cover and let stand 5 minutes. Whisk the mixture until smooth, then cover with plastic wrap and refrigerate until cool and slightly firm, 45 minutes to 1 hour (but not much longer).
Transfer the frosting to the bowl of the standing mixture and whip on medium speed until it is flurry and mousse-like and forms medium-stiff peaks, about 2-3 minutes. Set aside.