Preheat oven to 350 degrees.

Grease a 9x13 pan with non-stick butter spray.

In a medium size bowl, mix graham cracker crumbs, powdered sugar, and butter with a spoon.

Place mixture in the bottom of the baking pan, distributing it evenly and pressing it down firmly.

Bake for approximately 5-8 minutes (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.

Place in the oven for approximately 3 minutes (until chocolate just begins to melt).

Let cool for 1-2 minutes.

Pack marshmallows closely on top of the chocolate.

Turn oven to "broil" and put pan into oven for about 2 minutes, watching closely to avoid burning.

Let cool completely. Optionally, refrigerate to set further.

Cut and serve with a warm knife for easier slicing.