Position the oven rack in the lower third of the oven. Heat to 325. Line an 8x8 baking dish with foil so it hangs over the edges about 1 inch.
For the crust: Lightly butter the foil with some of the melted butter. Stir in the rest of the butter together with the crumbs, sugar, and salt. Press the crumb mixture evenly over the bottom of the pan. Bake until golden brown, about 20 minutes.
Meanwhile, make the brownie. Put the butter and chocolate in pan and melt over low heat. Stir the light brown and white sugars, vanilla, and salt into melted chocolate. Add the eggs and beat vigorously to make a thick, glossy batter. Add the flour and stir just until incorporated (do not overmix).
Pour the batter into the prepared pan. Bake until the top is crispy and toothpick inserted in middle comes out mostly clean, with a few crumps, about 35-40 minutes.
Remove from the oven and carefully position rack about 8 inches from broiler and preheat on low. Layer marshmallows across the top and toast until broiler until golden, or about 2 minutes. Keep an eye on this so they do not burn! Cool on rack, gently removing brownies from the pan using the aluminum flaps.
**These brownies are a little hard to cut because the marshmallows get sticky. Spray a sharp knife with cooking spray to cut these after they have cooled**.