What do you call the styrofoam-looking stuff they serve under the burgers at Island's?

This Island's?

http://www.islandsrestaurants.com/menu/b鈥?/a>

not a clue.

Or are you being sarcastic about their rolls?What do you call the styrofoam-looking stuff they serve under the burgers at Island's?
Most would think it was the plate, that they serve your burger on.