First, try to remove as much of the burned food as possible with a razor blade or spatula. If that doesn't work, you can try using a Glass and Ceramic Cooktop Cleaner. Be sure to follow the instructions on the cleaner and test it in an inconspicuous area first. You can also try making a paste out of baking soda and water and scrubbing the stain with that. Finally, if all else fails, you can try using sandpaper (fine grit) to gently sand away the stain.