Preheat your oven to 350°F (175°C) and grease a 9x13-inch baking pan.
In a large mixing bowl, combine the melted butter, sugar, and vanilla extract. Mix until well combined.
Add the eggs one at a time, mixing well after each addition.
In another bowl, whisk together the flour, cocoa powder, baking powder, and salt.
Gradually add the dry ingredients to the wet mixture, stirring until just combined.
Fold in the chocolate chips.
Pour half of the brownie batter into the prepared pan and spread evenly.
Sprinkle half of the graham cracker crumbs over the batter, followed by half of the mini marshmallows.
Pour the remaining brownie batter on top and spread it out.
Top with the remaining graham cracker crumbs, mini marshmallows, and drizzle with chocolate syrup.
Bake for 30-35 minutes, or until a toothpick inserted in the center comes out with a few moist crumbs.
Allow the brownies to cool in the pan for at least 15 minutes before slicing into squares.