You should be able to use a retractable razor blade to (very carefully!) scrape the burned residue off the stove top. We have had two glass top stoves, and both of them came with razors for this purpose. After you scrape off the residue, just clean/polish the stove top and it should be good as new. We've had some pretty nasty burn spots before, and usually this approach removes everything (or at least enough that your stove doesn't look horrible).