Preheat oven to 350F. Line a 9" x 9" baking pan with parchment paper, then set aside. 

In the bowl of a standing mixer fitted with the paddle attachment, beat the butter and sugars on medium speed until smooth and fluffy. Add egg and vanilla, mixing until combined. Use a rubber spatula to scrape the sides and bottom of the bowl to make sure everything is evenly distributed. 

In a medium-sized bowl, whisk together the flour, baking powder and salt. Gradually add to butter mixture and mix on medium-low until combined, then stir in the graham cracker crumbs. 

To assemble, press 2/3 of the dough into the prepared pan. Place chocolate bars on top, then create an even layer of marshmallow creme. With the remaining dough, use your hands to create thin, flat disks. place on top of the marshmallow layer until it's completely covered. You could also roll out this dough into one sheet on a non-stick surface, then place it on top. Whatever you find to be easiest! 

Bake for 20 – 25 minutes until top is golden brown. Let cool before serving. The more time it has to harden, the less messy it will be when they are cut into bars.