MICROWAVE baking bars and cream in medium, uncovered, microwave-safe bowl on MEDIUM-HIGH (70%) power for 1 minute; STIR. The bars may retain some of their shape. If necessary, microwave at additional 10 to 15-second intervals, stirring just until melted.
TRANSFER mixture to large mixer bowl; cool to room temperature. Beat in butter until creamy. Gradually beat in powdered sugar until light and fluffy. Makes 3 cups.