Preheat your oven to 350°F (175°C). Prepare a 9-inch pie dish or round baking pan by lightly greasing it with cooking spray or butter to prevent sticking.
In a large mixing bowl, combine the melted butter and granulated sugar. Whisk them together until you achieve a smooth and well-integrated mixture.
Add the eggs to the butter-sugar mixture one at a time, mixing thoroughly after each addition. Once all eggs are added, stir in the vanilla extract for flavor enhancement.
In a separate bowl, whisk together the all-purpose flour, unsweetened cocoa powder, baking powder, and salt until evenly combined. Gradually add this dry mixture to the wet ingredients, mixing gently until just combined—be careful not to over-mix.
Fold in the semi-sweet chocolate chips using a spatula, ensuring they are evenly distributed throughout the brownie batter for bursts of chocolate in every bite.
Pour half of the brownie batter into the prepared pie dish. Use a spatula to spread it evenly across the bottom.
Sprinkle the graham cracker crumbs evenly over the first layer of brownie batter, making sure to cover it entirely. Then, distribute the mini marshmallows on top of the graham cracker crumbs for that iconic s'mores flavor.
Gently pour the remaining brownie batter over the layer of marshmallows, using a spatula to smooth it out for an even top layer.
Bake in the preheated oven for 30-35 minutes. The brownies are done when a toothpick inserted into the center comes out with moist crumbs (aim for a fudgy texture, not wet batter).
Once baked, remove the pie from the oven and allow it to cool for approximately 10 minutes. Drizzle chocolate sauce on top while the pie is still warm to create a delightful, melting topping.
Slice into pieces and serve either warm or at room temperature.
Notes
For the ultimate s'mores experience, enjoy the gooeyness of the marshmallows and the rich brownie together!