Step 1: Preheat your oven to 350°F (175°C). 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: In a medium bowl, combine the graham cracker crumbs, ¼ cup granulated sugar, and ½ cup 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.
Step 3: For the brownie batter, in a large, heatproof bowl, combine 1 cup melted butter and 2 cups granulated sugar. Whisk vigorously for about 1-2 minutes until smooth and slightly thickened.
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: In a separate bowl, sift together the cocoa powder, all-purpose flour, baking powder, and salt. Gradually add the dry ingredients to the wet ingredients, mixing only until just combined. Do not overmix.
Step 6: Fold in 1 cup of semi-sweet chocolate chips. 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 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 the remaining ½ cup chocolate chips. The residual heat will melt them slightly.
Step 11: Let the Fudgy S'mores Brownies cool completely in the pan on a wire rack before lifting them out using the parchment paper overhang. Once completely cooled, slice into squares. Enjoy!