Microwave semisweet chocolate morsels and whipping cream in a 2-qt. microwave-safe bowl at MEDIUM (50% power) 2 1/2 to 3 minutes or until chocolate begins to melt. Whisk until chocolate melts and mixture is smooth. Whisk in butter, and let stand 20 minutes.
Beat at medium speed with an electric mixer 3 to 4 minutes or until soft peaks form.