Preheat oven to 350 degrees. Grease 9x13 pan with non-stick butter spray
In a medium size bowl mix graham cracker crumbs, brown sugar and butter with a spoon
Place mixture in the bottom of the baking pan. Distribute the mixture evenly and press down and firmly with your fingers to "pack it" along the bottom and sides of the pan.
Place in oven and bake for approx. 5 minutes (just until sides begin to get a bit golden brown)
Remove from oven and let cool.
Lay the Hershey bars on top of the graham cracker mixture - covering up all of the graham cracker as completely as possible.
Place in oven for approx. 3 minutes (until chocolate just begins to melt and gets shiny)
Let cool for 1-2 min.
Place marshmallows on top of the chocolate - pack them in their closely next to each other.
Turn oven to "broil" - put pan into oven and STARE at them! They only need to be in there for about 2 minutes. Watch them or they will burn. As soon as they begin to brown, take them out of the oven.
Let them cool completely.
Cut and serve - (tip: they can get really sticky to cut, warm a sharp knife under hot water, dry it off and immediately slice them).