Preheat oven to 350°F. In the bowl of an electric mixer, beat butter and sugar until light and fluffy. Beat in the egg and vanilla. Stir in graham cracker crumbs, flour, and salt.
Set one cup of the graham cracker mixture aside, and press the rest into the bottom of a greased 9×13″ pan. Layer chocolate on top, then sprinkle marshmallows over that. Take the remaining graham cracker mixture and crumble it all over the top in an even layer.
Bake for 20 to 25 minutes or until golden brown. Cool in the pan on a wire rack for 10 minutes. Cut into 24 bars and allow to cool completely.