Preheat and Prep: Preheat your oven to 350°F (175°C). Line a 9x13-inch baking pan with parchment paper, leaving an overhang for easy removal. Grease lightly.

Prepare Graham Cracker Base: Arrange the full-size graham crackers in a single layer on the bottom of the pan, breaking them as needed to fit.

Make the Brownie Batter: In a mixing bowl, whisk together the cocoa powder, sugar, vegetable oil, and melted butter until smooth. Add the eggs and vanilla extract, mixing until combined. Sift in the flour, baking powder, and salt. Stir until the batter is smooth and no lumps remain. Fold in the bittersweet chocolate chips for extra bursts of chocolate.

Layer the Brownies: Pour the brownie batter evenly over the graham cracker layer, smoothing the top with a spatula.

Bake the Brownies: Place the pan in the oven and bake for 25–30 minutes, or until a toothpick inserted into the center comes out with a few moist crumbs.

Add Marshmallow Topping: Remove the brownies from the oven and quickly scatter the mini marshmallows evenly over the top. Turn on your oven’s broiler setting and return the pan to the oven for 1–2 minutes, just until the marshmallows are golden and toasted. Keep a close eye to prevent burning.

Cool and Serve: Allow the brownies to cool completely in the pan. Use the parchment overhang to lift the brownies out and place them on a cutting board. Slice into squares and enjoy!