There's not much you can do. Taste it--if it has a hint of burned bitterness, you're just going to have to start over. When making butterscotch, it can go from nearly there to burned in a heartbeat. Telepan's suggestion to pour the hot icing into a separate saucepan is brilliant because it will cool it down fast and stop the cooking process. Using a candy thermometer is also a great way to ensure that you know when you're reaching the danger zone.