1. Microwave chocolate and 1/3 cup heavy cream in a microwave-safe bowl at MEDIUM (50% power) 1 to 1 1/2 minutes or until melted and smooth, stirring at 30-second intervals. (Do not overheat.) Let cool to room temperature (about 30 minutes).
2. Beat butter at medium speed with an electric mixer until creamy; gradually add powdered sugar and 1/4 cup heavy cream, 1 Tbsp. at a time, beating at low speed until blended after each addition. Beat in salt and chocolate mixture until light and fluffy. Stir in vanilla.