Position a rack in the lower third of the oven and heat oven to 325 degrees F. Line an 8 by-8-inch square baking pan with foil so it hangs over the edges by about 1 inch.
For the crust: Lightly butter the foil with some of the melted butter. Stir the rest of the butter together with the crumbs, sugar, and salt in a medium bowl. Press the crumb mixture evenly over the bottom of the pan. Bake until golden brown, about 20 minutes.
Remove from the oven and carefully position a rack about 6 inches from the broiler and preheat on low. Layer marshmallows across the top and toast under the broiler until golden, - I left them in 1 minute - watch carefully. Gently removing the brownies from the pan using the aluminum flaps. Cool on a rack for as long as you can stand to wait before cutting them and digging in.