Grease a 9" tart pan lightly with cooking spray and set aside. Meanwhile, preheat oven to 375 degrees F.

In a large bowl, combine the graham crumbs, white sugar and butter with a spatula until moistened and blended. Press the mixture evenly along the bottom and up the sides of the tart pan to form the crust. Bake for approx. 7 minutes on top of a baking sheet to help with easy removal, careful not to lift from the bottom of the tart pan. Cool completely.

In a large saucepan, combine the chocolate chips and sweetened condensed milk over medium-low heat, stirring constantly until smooth and melted. Immediately pour into the tart shell and very gently spread out into an even layer. Immediately top with the marshmallow bits and gently press to adhere. Allow the tart to set for about 1 hour before cutting into wedges to serve.