Preheat your oven to 350°F (175°C). Line a 9×13-inch baking pan with parchment paper, leaving some overhang on the long sides to lift the brownies out later. Lightly grease the parchment and sides with cooking spray or butter.

In a medium bowl, stir together the graham cracker crumbs and 1/4 cup granulated sugar. Pour in the 1/2 cup melted butter and mix until all the crumbs are evenly moistened and the mixture feels like wet sand. If it seems dry, add another tablespoon of melted butter.

Pour the graham mixture into the prepared pan. Press it down firmly with the bottom of a measuring cup or glass, making sure to get it into the corners. Bake for 8–10 minutes, until lightly fragrant and set. Set the pan aside while you make the brownie layer.

In a medium saucepan over low heat, melt 1 cup (2 sticks) unsalted butter. Once mostly melted, add the 8 oz of chopped semisweet or dark chocolate. Stir constantly until smooth and glossy. Remove from the heat and let it cool for 3–5 minutes so it does not scramble the eggs.

Whisk the 1 1/4 cups granulated sugar and 3/4 cup packed light brown sugar into the warm chocolate mixture. It will look thick and grainy at first. Add the eggs one at a time, whisking well after each addition, then stir in the vanilla extract. The batter should become shiny and slightly thicker.

In a separate bowl, whisk together the flour, cocoa powder, and salt. Sprinkle this dry mixture over the wet chocolate mixture. Gently fold with a spatula just until no dry streaks remain. Fold in the chocolate chips. Avoid overmixing to keep the brownies fudgy, not cakey.

Pour the brownie batter over the warm graham cracker crust and spread it evenly with a spatula. Bake at 350°F (175°C) for 22–28 minutes, or until the top is set and shiny and a toothpick inserted in the center comes out with a few moist crumbs but no wet batter.

Remove the pan from the oven and immediately sprinkle the mini marshmallows evenly over the hot brownie surface. Add extra chocolate chips or pieces if you like, plus a scattering of crushed graham crackers on top.

Return the pan to the oven for 3–5 minutes, watching closely, until the marshmallows are puffed and lightly golden. For a more toasted, campfire-style top, switch the oven to broil for 30–60 seconds, keeping the pan on a middle rack and watching constantly so the marshmallows do not burn.

Let the s'mores brownies cool in the pan for at least 60–90 minutes. For the cleanest slices, cool completely and chill for 30–45 minutes. Use the parchment overhang to lift the slab out of the pan, then cut into bars with a sharp knife, wiping the blade between cuts.