Preheat oven to 350 degrees F (176 degrees C). Line an 8x8 or 9x9 baking pan with parchment paper and set aside.
Beat the brown sugar and butter on medium speed until well combined. Add the egg and vanilla and mix just until combined. Add flour, graham cracker crumbs, baking powder, and salt. Mix just until combined.
Divide dough in half and press one half of the dough into the bottom of the pan. Lift the parchment paper and dough out of the pan and set aside (this is the top layer). Line baking pan with a clean piece of parchment paper and press remaining dough into the pan. Layer with chocolate bars, marshmallow creme, and top cookie layer.
Bake for 30 minutes or until lightly golden on top. Cool bars completely or even refrigerate if your house is warm for cleaner cut bars.