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 S'mores Brownies out later. Lightly grease the parchment paper.
Step 2: In a medium bowl, whisk together the graham cracker crumbs and ¼ cup melted butter until well combined. Press this mixture evenly into the bottom of the prepared baking pan. Bake for 8-10 minutes, until lightly golden. Remove from oven and let cool slightly while you prepare the brownie batter.
Step 3: In a large bowl, whisk together the 1½ cups flour, cocoa powder, baking powder, and salt. Set aside.
Step 4: In another large bowl, combine the 1 cup melted butter and granulated sugar. Whisk until smooth. Add the eggs one at a time, mixing well after each addition. Stir in the vanilla extract.
Step 5: Gradually add the dry ingredients to the wet ingredients, mixing until just combined. Be careful not to overmix the brownie batter; a few streaks of flour are okay.
Step 6: Pour the brownie batter evenly over the pre-baked graham cracker crust in the pan. Spread it gently to cover the entire surface.
Step 7: Bake for 22-25 minutes, or until the edges are set and a toothpick inserted into the center comes out with moist crumbs, but not wet batter. The brownies will not be fully baked yet. Remove from the oven.
Step 8: Evenly distribute the marshmallows over the top of the warm, partially baked brownie layer. If using large marshmallows, cut them in half and place them cut-side down for better coverage.
Step 9: Return the pan to the oven and bake for another 3-5 minutes, or until the marshmallows are puffed up and lightly golden brown. Keep a close eye on them as marshmallows can burn quickly. For a more toasted top, you can briefly place them under the broiler for 1-2 minutes, watching constantly.
Step 10: Once the marshmallows are perfectly toasted, remove the S'mores Brownies from the oven. Let them cool completely in the pan on a wire rack before lifting out with the parchment paper and cutting into squares. This allows the marshmallows to set, making them easier to cut cleanly. Enjoy!