From America's Test Kitchen
Makes about 2 1/4 cups
Cool the chocolate to between 85 and 100 degrees before adding it to the frosting. If the frosting seems too soft after adding the chocolate, chill it briefly in the refrigerator and then rewhip it until creamy. The frosting can be made up to 24 hours in advance and refrigerated in an airtight container. When ready to frost, place frosting in a microwave-safe container and warm briefly on high power until just slightly softened, 5 to 10 seconds. Once warmed, stir until creamy.