Preheat your oven to 350°F. Line a 9-inch baking dish with parchment paper and lightly coat it with baking spray.
In a large bowl, mix together the brownie mix and the ingredients called for on the box.
Spread half of the brownie batter into the prepared pan.
Arrange the S’mOreos over the batter in a 4×4 grid.
Carefully spread the remaining brownie batter over the cookies.
Sprinkle the milk chocolate chips evenly over the top.
Bake according to the brownie box directions, using the shortest time listed.
Pull the brownies out, sprinkle the mini marshmallows over the top, and return to the oven for another 3–5 minutes, until the marshmallows are soft and melty.
Let the brownies cool completely before slicing. (You can pop the pan in the fridge to speed things up—just bring them back to room temp before serving!)