Step 1: Preheat your oven to 350°F (175°C) and line a 9x13 inch baking pan with parchment paper, leaving an overhang on the sides to easily lift the brownies out later. Lightly grease the parchment paper.
Step 2: Create the graham cracker crust. In a medium bowl, combine the graham cracker crumbs, granulated sugar, and melted butter. Press this mixture evenly into the bottom of the prepared pan. Bake for 8-10 minutes, or until lightly golden. Remove from oven and set aside to cool slightly while you prepare the brownie batter.
Step 3: Prepare the fudgy brownie base. In a large, heatproof bowl, combine the melted butter and granulated sugar. Whisk vigorously for about 1-2 minutes until smooth and slightly thickened. This helps create that crinkly top!
Step 4: Add the eggs one at a time, beating well after each addition until fully incorporated and the mixture is glossy. Stir in the vanilla extract.
Step 5: Sift together the cocoa powder, all-purpose flour, baking powder, and salt in a separate bowl. Gradually add the dry ingredients to the wet ingredients, mixing only until just combined. Be careful not to overmix; overmixing can lead to tougher brownies.
Step 6: Fold in the chocolate chips or chunks. Pour the brownie batter evenly over the slightly cooled graham cracker crust in the prepared pan.
Step 7: Bake for 25-30 minutes, or until a toothpick inserted into the center comes out with moist crumbs attached (not wet batter). You want them slightly underbaked for ultimate fudginess.
Step 8: Remove the pan from the oven. Evenly sprinkle the marshmallows over the hot brownies. If using large marshmallows, you might want to cut them in half for better coverage.
Step 9: Return the Fudgy S'mores Brownies to the oven and bake for another 3-5 minutes, or until the marshmallows are puffed and beautifully golden brown. Keep a close eye on them, as marshmallows can burn quickly!
Step 10: Once the marshmallows are toasted, remove the pan from the oven and immediately sprinkle with additional chocolate chips (if desired). The residual heat will melt them slightly. Let the brownies cool completely in the pan on a wire rack before lifting them out using the parchment paper overhang.
Step 11: Once completely cooled (this is crucial for clean cuts!), slice into squares. A hot knife dipped in water and wiped clean between cuts will give you the cleanest edges for your Fudgy S'mores Brownies. Enjoy!