Preheat your oven to 350°F (175°C) and grease a 9x13-inch baking pan.
In a large bowl, mix the melted butter and sugar until well combined.
Add the eggs, one at a time, mixing well after each addition, then stir in the vanilla extract.
In another bowl, whisk together the flour, cocoa powder, salt, and baking powder.
Gradually add the dry ingredients to the wet ingredients, mixing until just combined.
Fold in the chocolate chips.
Pour half of the brownie batter into the prepared pan and spread it evenly.
Sprinkle half of the graham cracker crumbs over the batter, followed by half of the mini marshmallows.
Pour the remaining brownie batter over the top and spread it out.
Finish by sprinkling the rest of the graham cracker crumbs and mini marshmallows on top.
Bake for 25-30 minutes, or until a toothpick inserted into the center comes out with a few moist crumbs.
Allow the brownies to cool in the pan for at least 15 minutes before cutting into squares.