Preheat oven to 350 degrees. In a large bowl add flour, graham cracker crumbs, baking powder and salt. Whisk together and set aside. In a bowl of a stand mixer add in butter and brown sugar, cream until light and fluffy, about 3 minutes. Add in one egg at a time, making sure to fully incorporate before adding the next egg. Add vanilla extract and continue mixing until just incorporated. Stop the mixer and be sure to scrap down the sides of the mixing bowl well
Turn mixer back on to medium speed and mix for about 15 - 30 seconds making sure everything has been mixed well. Turn the speed down to low and slowly add in the dry ingredient mixture. Mix until just incorporated
In a parchment lined 9x13 baking pan, grease the parchment paper with the residual butter from the butter wrappers. Press 3/4ths of the cookie dough into the bottom of the pan until flat and smooth. Sprinkle in the bag of chocolate chunks, then layer the mini marshmallows on top. Dollop the remaining 1/4th cookie dough all over onto the top of the marshmallow layer, you want to see the marshmallows and chocolate chips show through on top
Bake in the oven for 30-35 minutes, the marshmallows and cookie dough should be golden brown. Remove from oven to a cookie rack and let completely cool. Trim the edges and cut into square shaped bars