Combine heavy cream and corn syrup in a microwave safe dish. Place in microwave and bring to a low boil. (alternatively, heat in a saucepan over your stove top).
Pour cream/corn syrup over chocolate. Let stand for 5 minutes, then whisk until smooth.
Add softened butter to bowl. Stir until smooth.
Allow icing to sit at room temperature and cool until it is firm enough to spread or pipe (approximately 1 hour).