Preheat the oven to 350°F (175°C). Line a 9×13-inch baking pan with parchment paper, leaving overhang on the long sides for easy lifting. Lightly spray the parchment and sides of the pan with nonstick spray.

In a medium bowl, combine the graham cracker crumbs, 3 tablespoons granulated sugar, 6 tablespoons melted butter, and a pinch of salt. Stir until the crumbs are evenly coated and the mixture resembles damp sand. Press firmly and evenly into the bottom of the prepared pan using the bottom of a measuring cup or glass.

Bake the crust at 350°F (175°C) for 8–10 minutes, until fragrant and just slightly darker at the edges. Set aside to cool slightly while you prepare the brownie batter.

In a medium saucepan over low heat, melt 10 tablespoons unsalted butter. When mostly melted, add 1 cup semi-sweet chocolate chips or chopped chocolate and stir gently until completely smooth and glossy. Remove from heat and let cool for 3–5 minutes so it does not cook the eggs.

Whisk the granulated sugar and light brown sugar into the warm chocolate mixture until fully combined; it will look thick and grainy at first. Add the eggs one at a time, whisking well after each addition, then whisk in the vanilla. The batter should look shiny and slightly thick.

In a separate bowl, whisk together the cocoa powder, all-purpose flour, fine sea salt, and baking powder. Sprinkle this dry mixture over the chocolate mixture and gently fold with a spatula just until combined. Fold in the remaining 1/2 cup chocolate chips for extra chocolate pockets.

Pour the brownie batter over the baked graham cracker crust. Use a spatula to spread it evenly to the edges and into the corners without pressing too hard, so you don’t disturb the crust.

Bake at 350°F (175°C) for 25–30 minutes, until the edges look set and a toothpick inserted into the center comes out with moist crumbs but not wet batter. If your oven runs hot, start checking around 22 minutes. Slight underbaking is better than overbaking for a fudgy texture.

Immediately after removing the pan from the oven, sprinkle mini marshmallows evenly over the top of the hot brownies, covering the surface in a thick single layer. If desired, sprinkle crushed graham crackers and additional chocolate chips over the marshmallows.

Return the pan to the oven and bake for 3–5 minutes, until the marshmallows are puffed and lightly golden. For a deeper toasted flavor, switch the oven to broil on high for 30–60 seconds, watching constantly so the marshmallows do not burn.

Place the pan on a wire rack and let the brownies cool for at least 60–90 minutes until set. For the cleanest slices, chill the cooled pan in the refrigerator for 30–45 minutes. Lift the brownies out using the parchment overhang and cut into bars with a sharp knife lightly greased or sprayed with nonstick spray, wiping the knife between cuts.