Preheat oven to 350 degrees. In a bowl, combine graham cracker crumbs, sugar, salt and butter. Reserve 1 cup of mixture and set aside. Press remaining mixture in bottom of 13X9X2-inch baking dish. Bake 12 minutes or until golden brown. Cool on a rack.
In a metal bowl set over a saucepan of barely simmering water, melt chocolate, stirring constantly. Spread evenly over baked crust; sprinkle with marshmallows , pressing them lightly. Sprinkle the reserved crumb mixturre evenly over all. Broil for 30 seconds. Cut into squares.