Preheat oven to 350°F. Spray a 9×13 pan with nonstick cooking spray and line with parchment.

Add 16 graham cracker squares to a large zip-top bag and crush with a rolling pin until you have fine crumbs (about 1 cup). Set aside.

Melt 1 cup unsalted butter in a microwave-safe bowl for 30-60 seconds until just melted. Transfer to a large bowl and whisk in 1 cup light brown sugar and ½ cup granulated sugar for 1 minute until combined.

Mix in 2 large eggs and 2 teaspoons vanilla extract. Add the 1 teaspoon baking soda, ½ teaspoon baking powder, and ½ teaspoon salt and mix again. Stir in 2 ½ cups all-purpose flour and graham cracker crumbs until no flour remains. Fold in all the 1 cup semi-sweet chocolate chips.

Drop half of the cookie dough in small chunks across the prepared pan. Press and smooth into an even layer, making sure to reach all the corners.

Lay the remaining 20 graham cracker squares in a single layer over the dough, arranging them to cover as much surface as possible.

Dollop 13 ounces marshmallow fluff over the graham cracker layer, then spread it gently into an even layer. Tip: lightly spray your spatula with cooking spray to prevent sticking.

Take the remaining cookie dough and flatten small portions between your fingers into thin discs. Drop and place them over the marshmallow fluff layer, covering as much of the surface as possible while still leaving some 'holes' of marshmallow fluff. Pinch and press edges together where you can.

Bake at 350°F for 18-20 minutes or until the marshmallow fluff just starts to turn a light golden brown. Do not overbake. After removing from the oven, press on pieces of 1 Hershey bar.

Allow to cool completely in the pan before cutting into 24 bars. The marshmallow layer sets as it cools and makes cutting much cleaner.