Remove white stain from countertop
That appears to be Corian (or something similar).  You can find cleaning and repair guides on the DuPont website.  These counter tops are not horribly heat resistant (as you found out), and can discolor or even melt it enough heat is applied.  

I've heard that letting baking soda sit on the stain for a while will remove some discolorations, but I've never tried it myself.  My guess is that you'll have to try to polish it out with a very mild abrasive and hope it didn't go too deep.  See the section in the linked page above for "Removing Fine Scratches".