Prepare a 9"x13” baking dish by coating with butter and lining with parchment paper. Preheat oven to 350ºF.
In a medium bowl, combine flour, graham cracker crumbs, baking soda, and salt.
In a stand mixer or large bowl, beat butter and brown sugar together until light and fluffy.
Add eggs and vanilla, beating until well combined. Add flour mixture and beat on low speed until blended.
With a spatula, press half of dough into the bottom of baking pan. Dollop marshmallow fluff on top and gently spread into an even layer. Tip: Spray spatula with cooking spray to prevent sticking.
Sprinkle on chocolate chips, then small pieces of remaining dough on top. Gently press down to create an even layer.
Bake for 28-30 minutes, or until golden brown. Allow to cool completely before cutting and serving. Enjoy!