Preheat the oven to 325℉. Line an 8x8-inch cake pan with 2 sheets of parchment paper so it hangs over all 4 sides. Spray the bottom and sides with nonstick spray.
Add the mini marshmallows, chopped graham cracker, and chopped chocolate to a large bowl and toss gently.
In a medium saucepan, combine the butter and sugar over medium-low heat. Heat until the butter is melted and the sugar is fully dissolved, stirring frequently. Pour the butter mixture into a large bowl and let it cool for 30 seconds to 1 minute. Whisk in the vanilla and salt, and then the eggs. Sift in the cocoa powder and all-purpose flour, and stir to combine. Stir ½ of the S’mores Mixture into the batter.
Transfer the batter to the prepared pan, and spread it out evenly. Bake 20 minutes. Remove the brownies from the oven and carefully spread the remaining ½ of the S’mores Mixture on top. Return the brownies to the oven and bake until a toothpick inserted in the center comes out a little moist, about 5 to 10 minutes longer. To brown the marshmallows, briefly broil the brownies once they’re done baking.
Let the brownies cool before cutting.
Notes
Line the pan with parchment for easy removal
Watch the broiler carefully—marshmallows can burn quickly
Let brownies cool fully before slicing for clean cuts