Preheat oven to 325*F and line a 9"x13" baking pan with parchment paper (or foil). Leave some paper hanging over the sides so you can easily pull the bars out of the pan after baking.
Crush graham crackers, then combine 1/2 cup melted butter with crumbs and stir until well combined. Pour mixture into prepared pan, and press evenly into the bottom of the pan. Put entire pan into the freezer while you make the cookie dough.
In a large mixer, blend together both sugars and the melted butter. Add eggs (one at a time) and vanilla and continue to blend until well combined.
In a separate bowl, add flour, baking powder and salt. Then, while the mixer is on low, slowly add the flour to the butter/sugar mixture. Stop the mixer immediately when you have added all the flour. Add the marshmallows and chocolate chips and blend until all of the ingredients are well combined.
Retrieve the baking pan from the freezer and scoop the dough on top of the prepared crust. Use an offset spatula to carefully spread until evenly distributed.
Top the dough with the toppings listed, pressing them into the dough slightly.
Bake 35-45 minutes, or until the top is golden brown, and there is no wet batter visible.
Remove from oven and allow to cool FULLY before cutting (1 hour or more). For easiest cutting, run a knife around any edges that are touching the sides of the pan, then pull up on the parchment paper and place giant bar on a cutting board.