Instructions

1. Prepare: Preheat oven to 350º F. Line an 8 x 8 inch baking pan with parchment paper. Set aside. 
2. Make Graham Cracker Layer: In the bowl of a stand mixer, combine the butter, brown and granulated sugars and beat on medium until well combined, about 1-2 minutes. Add in egg and vanilla and beat again on medium for 1 minute, scraping the sides and bottom of bowl, as needed. With mixer off, add in flour, baking soda, salt, and graham cracker crumbs. Stir until combined. Do not overmix. Press graham cracker mixture into bottom of prepared baking dish
3. Bake Graham Cracker Layer: Bake in preheated oven for 15 minutes or until edges of graham cracker layer are just beginning to turn golden brown and the middle looks set. Remove from the oven and place chocolate gently on top of graham cracker layer to cover graham layer completely. I was able to fit two full chocolate bars side-by-side, then break the 3rd bar into pieces to fill the remaining space in an even, single layer. If using the squares, approximately 16 should fit nicely in one layer across the top of the graham layer. Next, place 16 marshmallows on top of chocolate bars, if using. If using marshmallow fluff, do not add until after bars have baked for another 15 minutes (see next step)
4. Top them off: Return pan to the oven and bake for 15 minutes. Remove from oven and turn on the broiler. If using marshmallow fluff, carefully spread fluff on top of chocolate in a thick layer. For both marshmallows and marshmallow fluff, return to oven and broil for 1 minute. Watch carefully to make sure the marshmallows do not burn. Remove from oven and allow to cool for 10 minutes. Using the parchment paper overhang, carefully lift bars out of the pan and cut into 16 squares