Prepare the Baking Dish:

• Preheat the oven to 325°F (165°C).

• Line a 9×13-inch glass baking dish with parchment paper, leaving an overhang on two sides for easy removal. Lightly spray with non-stick spray to secure the paper.

• Tip: Use a glass dish for even baking, and ensure parchment covers the sides to prevent sticking.

Prepare the Brownie Batter:

• Prepare 1 box brownie mix according to package instructions (typically mixing with eggs, oil, and water) in a large mixing bowl, stirring until smooth, about 1–2 minutes.

• Tip: Follow the box’s “fudgy” instructions for a denser texture, and mix until just combined to avoid tough brownies.

Layer the Graham Crackers:

• Lay 7–9 full graham cracker sheets in a single layer across the bottom of the prepared baking dish, breaking pieces to fit and cover as much surface as possible.

• Tip: Arrange crackers tightly to form a solid base, and trim edges with a knife for a neat fit.

Add the First Brownie Layer:

• Using a spatula, spread about half of the brownie batter over the graham crackers, covering them as evenly as possible, about 1–2 minutes.

• Tip: Spread gently to avoid shifting the crackers, and use an offset spatula for a smooth layer; reserve the remaining batter.

Layer the Mini Oreos:

• Place 8 oz Mini Oreos in a single layer over the brownie batter, arranging them closely to cover the surface, about 1 minute.

• Tip: Press Oreos lightly to adhere to the batter, and use full-sized Oreos if preferred, spacing them evenly (about 12–15 cookies).

Add the Marshmallow Layer:

• Cut 20 flat s’more marshmallows (such as Jet-Puffed S’moreMallows) in half to create thinner pieces, yielding 40 halves.

• Layer the marshmallow halves evenly over the Oreos, covering the cookies as much as possible, about 1–2 minutes.

• Tip: Cut marshmallows with a greased knife to prevent sticking, and overlap slightly for a uniform layer.

Cover with Remaining Batter:

• Spread the remaining brownie batter over the marshmallows and around the sides of the Oreos, using a spatula to smooth it evenly, about 1–2 minutes.

• Tip: Work gently to cover the marshmallows completely, and spread to the edges for a polished look; tap the dish to level the batter.

Bake:

• Bake in the preheated oven for 45 minutes, or until the top is set and a toothpick inserted in the center comes out with a few moist crumbs.

• Check for doneness around 35 minutes, as ovens vary; the brownies should be slightly firm but fudgy.

• Tip: Slightly overbake (45–48 minutes) for easier slicing, and cover with foil if the top browns too quickly.

Add the Topping:

• Remove the brownies from the oven and immediately top with the remaining marshmallow halves (from the 20 cut marshmallows, about 10–12 halves).

• Place the dish under the broiler on high for 1–2 minutes, until the marshmallows are golden brown, watching closely to prevent burning.

• Tip: Stay by the oven during broiling, as marshmallows burn quickly; rotate the dish if needed for even toasting.

Cool and Slice:

• Let the brownies cool completely in the dish on a wire rack, about 2 hours, to set and avoid a gooey mess when slicing.

• Use the parchment overhang to lift the brownies out and transfer to a cutting board.

• Slice into 12–16 squares (3×4 or 4×4 grid) using a sharp knife, wiping the blade between cuts for clean edges.

• Serve at room temperature, optionally with ice cream or hot cocoa.

• Tip: Run the knife under hot water for smoother cuts, and refrigerate for 30 minutes before slicing if still too gooey.