Preheat oven to 350 degrees. In a medium-size bowl combine the melted butter with the crushed graham cracker crumbs, sugar, flour, vanilla and salt. Press into a 9 x 13 baking pan that has been sprayed with nonstick cooking spray. Sprinkle the chocolate chips all over the top of the gram cracker crust and continue with marshmallows on top. Bake for 20 minutes and then broil for a few minutes to brown the marshmallows. Remove from the oven and cool. Cut into bars and store in an airtight container.