Beat first 3 ingredients at medium speed with an electric mixer until creamy. Whisk together powdered sugar and cocoa in a medium bowl; gradually add to butter mixture alternately with 5 Tbsp. cream. Beat at low speed until blended after each addition. (If needed, add up to 1 Tbsp. cream, 1 tsp. at a time, to reach desired consistency.) Increase speed to medium, and beat 1 to 2 minutes or until light and fluffy.