A 9-by-13-in. pan makes flat rectangles ideally sized for the Ultimate S'more, but you could also use a 9-in. square pan--the marshmallows will be thicker, though. The recipe comes from Cindy Pawlcyn, chef-owner of Mustards Grill in Napa as well as Cindy's Backstreet Kitchen and Go Fish (both in St. Helena, California).
Mustards Grill in Napa; Cindy's Backstreet Kitchen and Go Fish, both in St. Helena, CA