Preheat oven to 350°F (175°C).
Grease a 9x13 inch baking pan with non-stick spray or butter.
In a large bowl, mix graham cracker crumbs, powdered sugar, and melted butter until combined.
Press the mixture into the bottom of the prepared pan.
Bake the crust for 8-10 minutes or until it's lightly golden brown.
Remove the pan from the oven and layer the broken chocolate bars evenly over the crust.
Place the pan back in the oven for 3-5 minutes until chocolate melts.
Remove from oven and sprinkle marshmallows on top of the melted chocolate.
Turn the oven to broil and carefully put the pan back in for 1-2 minutes.
Once browned, remove the pan from oven and let cool completely.
Cut into bars and serve.