Preheat oven to 350 degrees F. Lightly grease a 13x9-inch baking dish with shortening or nonstick spray.
In a medium bowl, mix together flour, graham cracker crumbs, baking soda, and salt until well combined.
In a separate large bowl, mix together butter and sugars with an electric mixer on medium speed until well combined. Mix in egg and vanilla until well combined then flour mixture.
Spread half of cookie batter on bottom of greased baking dish using your fingers or back of a fork. Top with chocolate bars in a single layer. Break chocolate bars as needed to cover cookie dough to sides of pan. Spread marshmallow fluff on top of chocolate.
Take remaining cookie dough and spread in a thin even layer on a piece of parchment paper (or wax paper or silicone mat) to form a rectangle about 13x9 inches. Place dough on top of marshmallow fluff with parchment paper side up. Carefully peel back parchment paper and discard. With your fingers fix any holes in the top layer of cookie dough.
Bake cookie bars for about 25 to 30 minutes or until golden brown around edges.
SNAPPY TIPS: Instead of flattening the top cookie layer on a piece of parchment paper, you can take small amounts of dough and flatten with your hands then place on top of marshmallow fluff covering as much of the marshmallow as possible. Don't worry if the marshmallow is not covered 100%!SNAPPY SUBSTITUTIONS: If you have to, you can substitute king size chocolate bars or chocolate chips. The marshmallow fluff can be difficult to spread over chocolate chips though. Do NOT substitute regular marshmallows for the marshmallow fluff. Marshmallows will melt in the oven and the cookie bars will not turn out well.