Couple of options:
Use CERAMA BRYTE Ceramic Cooktop Cleaner ($19) - Readily Biodegradable
Quickly and safely removes stains from glass ceramic cooktops. Can be used on ALL ceramic cooktops.

From Maytag.com
Porcelain Cooktops May etch or discolor when a spillover that is high in sugar or acid, such as tomato and milk based foods, occurs. These spills should be wiped up immediately with a dry cloth. (Never wipe a hot porcelain surface with a wet cloth as cracking and chipping may result.)

From eHow.com
1. Use a razor blade to carefully scrape at any food that is crusty or burned on to the cooktop.
2. Soak a sponge in vinegar and apply to the stain, leave overnight and re-treat if necessary.
3. Treat the spot with a regular steam cleaner if the vinegar is unable to remove the stain completely.
4. Use a cleaner that is specified for ceramic (such as Cerama-Bryte) to clear any residue. Dry with a soft cloth.
5. Treat the cooktop with a ceramic specified conditioner such as Cera-Fix to guard against future stains.
Source(s):
http://www.maytag.com/content.jsp?name=troubleshoot_cooktops_advice#glass-c...

http://www.ehow.com/how_4811290_remove-stains-ceramic-cooktop.html

http://www.ceramabryte.com/productDetail.asp?SID=B67FE04C6D464F7AA307CD5E14...

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