For the crust:
6 tablespoons unsalted butter, melted and cooled
1 1/2 cups graham cracker crumbs (about 10 full crackers)
2 tablespoons sugar
1/4 teaspoon salt
For the brownies:
8 tablespoons (1 stick) unsalted butter
7 ounces bittersweet chocolate
1 cup all-purpose flour
1/4 teaspoon salt
2 large eggs
1/2 cup sugar
1/2 cup brown sugar
1 teaspoon vanilla extract
1 10-ounce bag large marshmallows
To make the crust: Preheat the oven to 325°F. Line an 8x8-inch pan with parchment and set aside.
In a medium bowl, combine the graham cracker crumbs, sugar, and salt, and mix together. Pour in the melted butter and stir until well combined and all the crumbs are evenly coated with butter. Pour the crumbs into the pan, and use a fork to press evenly along the bottom and sides. Bake for 20 minutes. Remove from the oven and set aside.
To make the brownies: Increase the oven temperature to 350°F and prepare the brownie layer. In a medium saucepan over low heat, add the butter and chocolate, and melt, stirring constantly. Remove the pan from the heat and cool completely.
In a medium bowl, whisk together the flour and salt. Set aside.
In a stand mixer fitted with a paddle attachment, beat the eggs and sugars until fluffy, 2 to 3 minutes. Beat in the vanilla extract. Gently mix in the melted chocolate. Fold in the flour.
Evenly pour the brownie batter over the cooked graham cracker crust. Transfer the pan to the oven and cook for 23 to 25 minutes. Remove the pan from the oven.
To make the marshmallow layer: Turn the oven to broil. Arrange the marshmallows over the top of the baked brownies. Then place the pan on the lower rack of the oven and broil for 1 to 2 minutes, until the marshmallows are melted and light brown on top — 30 seconds to 1 minute for golden marshmallows, and 1 1/2 to 2 minutes if you prefer more charred marshmallows.
These are best served immediately. Though, they will last for 3-4 days in a foil-covered pan.