Preheat oven to 350° F. Grease and line a 9-by-9 inch baking pan with parchment paper, set aside.
In a medium bowl, whisk together the flour, baking powder, and salt.
In the bowl of a standing mixer, whip the butter and sugars until light and fluffy.
Add in the egg and vanilla extract. Mix until just incoporated.
Pour the dry ingredients into the butter mixture, mix until well incporated. Fold in the ½ cup graham cracker crumbs.
Press half of the cookie dough into the prepared baking dish, smoothing (pressing) into one even layer. Sprinkle ¾ of the package of chocolate chunks over the cookie dough layer. Lightly press into the dough, making sure chocolate is evenly spread out.
Lightly grease a spatula with cooking spray. Using the greased spatula, pour the marshmallow fluff from the jar onto the cookie layer. Spread evenly into one layer. (See notes above in post.)
Using a spoon or cookie scoop, scoop small balls of dough onto the marshmallow layer. Using the back of your spoon, or with your fingers, flatten the dough balls down so that most of the marshmallow layer is covered.
Sprinkle the 2 tbsp of graham cracker crumbs over this cookie layer and gently press in.
Bake for 30 minutes, or until golden brown. Allow to cool before cutting into slices.