1 12-ounce box vanilla-wafer cookies
2 1/4 cups sugar, divided
3/4 teaspoon kosher salt, divided
1 cup plus 5 tablespoons unsalted butter, melted, divided
5 1-ounce squares unsweetened chocolate, melted
1 teaspoon vanilla extract
5 large eggs, at room temperature
1 cup all-purpose flour
1 16-ounce bag large marshmallows, cut in half
Preheat oven to 350°. Line a 13x9-inch pan with aluminum foil, letting foil hang over sides of pan. Set aside. In the work bowl of a food processor, pulse together cookies, 1/4 cup sugar, and 1/4 teaspoon salt until mixture is fine. With processor running, slowly add 5 tablespoons melted butter, and continue blending until mixture is well combined. Press crumb mixture firmly into the bottom of prepared pan.
In a large bowl, whisk together remaining 1 cup melted butter and melted chocolate until mixture is well combined. Whisk in remaining 2 cups sugar and vanilla. Add eggs one at a time, whisking constantly, until mixture is smooth and glossy. Whisk in flour and remaining 1/2 teaspoon salt. Pour batter into pan, smoothing top.
Bake until a wooden pick inserted in the center comes out with just crumbs, about 30 minutes. Let cool slightly.Preheat oven to broil. Place marshmallow halves on top of brownies, in rows, as close as possible. Broil until tops of marshmallows are golden brown and puffed.you may have to rotate the pan a couple of times to get even browning. Let cool slightly before cutting brownies into squares. Use a sharp knife to cut brownies. Dipping the knife in a glass of warm water between cuts helps it not to stick to the marshmallows.