In a 1 1/2- to 2-quart pan over high heat, bring cream and corn syrup to a boil, stirring occasionally, 3 to 4 minutes.
Put chocolate in a heatproof bowl. Pour hot cream mixture over chocolate. Let stand about 5 minutes, then stir until smooth. Add butter and stir until melted and mixture is smooth.
Let frosting cool, stirring occasionally, until thick and spreadable, 2 to 3 hours at room temperature, about 1 1/2 hours in the refrigerator.
Note: Nutritional analysis is per tablespoon.