Position a rack in the lower third of your oven and preheat to 325 degrees F. Line an 8″ x 8″ square baking pan with foil so it hangs over the edges by about an inch.

For the crust: lightly butter the foil with some of the melted butter. Stir the rest of the butter together with the graham cracker 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.

Meanwhile, make the brownie. Put the butter and chocolate in a medium microwave safe bowl. Using a double boiler set-up, put the butter and chocolate in a large heatproof glass bowl. Bring water up to a simmer; set the bowl on the pan. Stir occasionally until melted. Stir in the brown sugar, white sugar, vanilla, and salt into the melted chocolate. Add the eggs and beat vigorously to make a thick and glossy batter. Add the flour and stir until just incorporated.

Pour batter into the prepared pan. bake until the top is crispy and a toothpick inserted in the center comes out clean, about 40 to 45 minutes.

Remove from the oven and carefully position a rack about 6 inches from the broiler and preheat oven on low broil. Layer marshmallows across the top and toast under the broiler until golden, (keep an eye on it, burning can happen quickly), about 2 minutes. Cool on a rack, gently removing the brownies from the pan using the foil flaps. Carefully separate any marshmallow from the foil and fold away. Cut into 12 – 2″ squares.