Microwave chocolate morsels and 1/4 cup whipping cream in a 1-quart microwave-safe bowl at MEDIUM (50% power) 3 1/2 minutes. Stir until smooth.
Beat butter at medium speed with an electric mixer until fluffy; gradually add 3 cups powdered sugar, beating well. Gradually add chocolate mixture and vanilla, beating until smooth.
Add remaining 7 cups powdered sugar alternately with remaining 1/2 cup whipping cream, beginning and ending with powdered sugar. Beat mixture at low speed until well blended after each addition.