These S'mores Cookie Bars are made with a sweet graham cracker crust, cookie dough with marshmallows folded in, and topped with Hershey bar squares for that distinct s'mores taste.
Preheat the oven to 350 degrees F (165 degrees C). Place marshmallows in freezer if you haven't already (they don't take long to freeze). Spray a 9x13 baking dish with cooking spray.
For the graham cracker crust: In a medium bowl, combine the crumbs,sugar, and butter until combined. Press into the prepared baking dish.
Sift together the flour, baking soda and salt; set aside.
In a medium bowl, cream together the melted butter (make sure it's mostly cooled), brown sugar and white sugar until well blended. Beat in the vanilla, egg, and egg yolk until light and creamy. Mix in the sifted ingredients until just blended. Stir in the chocolate chips and 1 cup of the frozen marshmallows.
Scoop dough on top of the graham cracker crust and gently press in an even layer. Don't worry about filling the pan too much - the dough will cover all of it as it bakes. Sprinkle remaining 1/2 cup of marshmallows on top slightly pressing in.
Bake for 18-25 minutes or until top begins to get touches of golden brown. Make sure you bake until the center is set and not jiggly. They will continue to firm up after cooling so be sure not to overbake.
After bars are done baking, remove pan from oven and let sit for about 5 minutes then carefully place broken candy bar pieces on top. Let cool completely before cutting into squares.