Preheat oven to 350°F. Line an 8-inch-square metal baking pan with foil and spray lightly with nonstick cooking spray.
Combine the graham cracker crumbs, melted butte, and granulated sugar in a bowl and mix until the crumbs are moistened. Press the crumbs evenly into the bottom of the pan and bake for 7 minutes. Allow crust to cool on wire rack.
Turn the oven temperature up to 400°F and prepare the brownie layer.
In top of a double boiler set over barely simmering water, or on low power in a microwave, melt butter and chocolate together. Stir often, and remove from heat when a few lumps remain. Stir until smooth.
Stir in sugar, vanilla and salt. Stir in eggs one at a time, followed by flour. Stir until very smooth, about 1 minute, until mixture pulls away from sides of bowl. Scrape batter on top of cooled crust, smooth the top, and bake for 18 minutes.
Let cool completely, then lift out using the foil and place brownies onto a broiler-safe pan. Move your oven rack about 6 inches from your broiler and preheat your broiler to low.
Layer marshmallows (sticky side down) across the top and toast under the broiler until golden, (watch this like you have never watched anything before - it can burn QUICK), about 2 minutes. Cool and cut into 16 (two inch) squares.