Preheat oven to 350 degrees. Line a 9″ square pan with parchment and spray with baking spray.
Make the brownies according to the package directions, and place the batter into the pan, spreading it smooth with a spatula.
Top it with an even layer of S’moreos, pressing into the batter.
Bake it for 30-35 minutes. If the brownies are not fully baked, be sure to leave it a bit longer.
Let them cool for approximately 30 minutes.
Melt the chocolate and pour it on top of the brownies, spreading it into an even layer. Top with crushed cookies and mini marshmallows.
Let the brownies sit on a wire rack until completely set. To speed up the process and for a chewier brownie, refrigerate them for about an hour. Serve at room temp so that the chocolate is easy to bite through.