Preheat the oven to 350 degrees. Grease a 9x13 baking dish with non-stick cooking spray. In a large mixing bowl, mix together the graham cracker crumbs, melted butter, and the powdered sugar with a spoon or fork.
Dump the crumb mixture into the greased baking dish and spread it out evenly and press it down firmly with your fingers. You want to at least cover the entire bottom and a little of all 4 sides. Place the baking dish in the oven for 5-8 minutes. Remove the dish from the oven and let it cool. The crust will firm up as it cools so don't worry that it hasn't baked long enough just yet.
Lay the extra large chocolate bars on top of the graham cracker layer, covering the graham cracker layer as much as possible. You can break your bars down to the smaller size pieces if you prefer. Return the baking dish to the oven and bake for about 2-3 minutes or until the chocolate just begins to look shiny and melty. Remove the dish again and let it cool a couple of minutes.
Layer the mini marshmallows on top of the semi melted chocolate layer, making sure that the marshmallows are as packed in the dish as you can get them covering the entire chocolate layer. Change the oven setting over to "broil" and place the s'mores bars back in the oven one more time. Watch them this time like a hawk. They only need to be in there a minute or so and they will be done. Trust me, any longer than that and you will have a baking dish to put out.
Remove the dish of s'mores bars and let them cool. Once the dish has cooled completely off you can sit the s'mores bars in the fridge for a couple of minutes to continue cooling.
Run your cutting knife under hot water to warm it and slice the s'mores bars cold which should help cut through the marshmallow layer easier.