Preheat oven to 400 degrees (F).
Create a foil sling for a 9-inch square baking dish. Fold 2 long sheets of foil to fit the inside of the baking dish then place them perpendicular to each other, allowing extra foil to hang over the edge of dish. Spray foil with non-stick cooking spray.
Using an electric mixer, cream together the butter with the granulated and brown sugars, and salt, about 2 minutes. Beat in the egg and vanilla.
Stir in the flour, graham cracker crumbs, and baking powder. The mixture will be crumbly.
Reserve 1 cup of the dough and press the remaining dough into the prepared baking dish to form the crust.
Cover the crust with 5 chocolate bars, breaking bars as necessary to cover evenly. (Try to leave the bars as whole as possible.) Coarsely chop the remaining chocolate bar and set aside.
Spray a spatula with non-stick cooking spray and use to spread the marshmallow fluff over the chocolate bars.
Sprinkle the marshmallow fluff with mini marshmallows and the reserved cookie dough mixture.
Bake for 22 - 24 minutes or until the top looks golden brown. Remove the dish from the oven and sprinkle with the chopped chocolate bar.
Allow the bars to completely cool until the chocolate has set, about 4 hours. Do not refrigerate.
Using the foil sling, remove the S'mores Cookie Bars from the dish and place on a chopping board. Peel off the foil and cut into bars with a knife spritzed with non-stick cooking spray.
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