Preheat and Prep. Preheat your oven to 350°. Grease an 8x8 inch baking pan or line it with parchment paper for easy removal.
Warm and Combine Wet Ingredients. Melt the butter in a medium saucepan over medium heat. Remove from heat and stir in sugar, eggs, and vanilla until well combined.
Mix In Dry Ingredients. Beat in cocoa, flour, baking powder, and salt. Spread the batter evenly into the prepared baking pan.
Top with Graham Crackers, Chocolate Chips, and Marshmallows. Sprinkle the broken graham crackers evenly over the top of the batter, pressing them slightly into the mixture. Then, distribute the mini marshmallows and chocolate chips evenly over the graham crackers.
Bake the Brownies. Bake in the preheated oven for 25 to 30 minutes, or until a toothpick inserted into the center comes out with a few moist crumbs. Be careful not to overbake, as the brownies should be moist and fudgy.
Garnish with Additional Toppings. Remove the brownies from the oven and immediately sprinkle the additional graham cracker pieces, mini marshmallows, and chocolate chips on top. The heat from the brownies will slightly melt the toppings, creating the s'mores effect.
Cool and Cut. Allow the brownies to cool in the pan on a wire rack before cutting them into squares. For cleaner cuts, use a sharp knife and wipe it clean between cuts.
Serve and Enjoy! Serve and enjoy your homemade S'mores Brownies! These brownies are best enjoyed warm, with the marshmallows gooey and the chocolate chips melted.