Prepare the Baking Pan: Begin by preheating the oven to 350°F (175°C). Line an 8x8 inch baking pan with parchment paper so the brownies lift out easily later. Lightly grease the paper as well to prevent sticking. Preparing the pan first makes the baking process smoother once the batter comes together.
Make the Graham Cracker Crust: Combine graham cracker crumbs, melted butter, and sugar in a bowl. Stir until the mixture resembles damp sand and holds together when pressed. Spread this mixture evenly across the bottom of the prepared pan, then press it down firmly with the back of a spoon or measuring cup to form a solid crust layer.
Pre-Bake the Crust: Place the crust in the oven and bake it for about 8 minutes. This short baking time helps the graham cracker base firm up slightly and prevents it from becoming soggy later. Once done, remove the pan and allow the crust to cool for several minutes while preparing the brownie batter.
Mix the Brownie Batter: In a large mixing bowl, whisk together melted butter and sugar until the mixture looks glossy and smooth. Add the eggs and vanilla extract, then whisk again until everything blends evenly. The batter should appear thick and shiny at this stage.
Add the Dry Ingredients: Stir cocoa powder, flour, salt, and baking powder into the bowl. Mix gently until the batter becomes smooth and fully combined. Avoid overmixing, since that can make brownies slightly tougher instead of soft and fudgy.
Layer the Brownie Batter: Pour the prepared brownie batter over the graham cracker crust. Spread it evenly with a spatula so the surface stays level. The batter should completely cover the crust and reach the corners of the pan.
Bake the Brownies: Place the pan back into the oven and bake for about 20 minutes. The center should look slightly soft but mostly set. Brownies continue cooking slightly after removal, so avoid overbaking at this stage.
Add the S’mores Topping: Remove the pan carefully and sprinkle mini marshmallows across the surface. Scatter chocolate chips and broken graham cracker pieces over the marshmallows. This layer creates the signature s’mores flavor once toasted.
Finish Baking: Return the pan to the oven for another 5 to 7 minutes. During this time the marshmallows puff up and turn lightly golden while the chocolate begins melting. Watch closely so the marshmallows toast without burning.
Cool Before Slicing: Allow the brownies to cool in the pan for at least 20 minutes. This resting time helps the layers set and makes slicing easier. Once cooled slightly, lift the brownies out using the parchment paper and cut them into squares.
