Preheat your oven to 350°F (175°C). Lightly grease a 9-inch pie dish or a round baking pan.
In a large mixing bowl, combine the melted butter and granulated sugar, stirring until well mixed.
Add the eggs, one at a time, mixing thoroughly after each addition. Stir in the vanilla extract.
In a separate bowl, whisk together the flour, cocoa powder, baking powder, and salt. Gradually add the dry ingredients to the wet ingredients, mixing until just combined.
Fold in the semi-sweet chocolate chips until evenly distributed throughout the batter.
Pour half of the brownie batter into the prepared pie dish, spreading it evenly.
Evenly sprinkle the graham cracker crumbs over the brownie batter layer. Then, add the mini marshmallows on top of the graham cracker crumbs.
Pour the remaining brownie batter over the marshmallows, smoothing it out with a spatula.
Bake in the preheated oven for 30-35 minutes, or until a toothpick inserted comes out with moist crumbs (but not wet batter).
Remove the pie from the oven and let it cool for about 10 minutes. Drizzle chocolate sauce on top while still warm for an extra indulgent treat.
Cut into slices and serve warm or at room temperature, allowing the marshmallows to stay gooey for that classic s'mores experience.