Preheat oven to 350 degrees F. Line a 13x9" baking pan with foil, extending the sides of the foil over the edges of the pan. Mist the foil with cooking spray and set aside.

In a large bowl, combine the cake mix, egg and oil until a soft dough forms. Press the dough evenly into the prepared pan and bake for approx. 12 minutes.

Remove the pan from the oven, but keep the oven on. Top the crust evenly with the chocolate chips, white chips, Oreo cookie pieces and Hershey's bars. In a small bowl, whisk together the sweetened condensed milk and the marshmallow fluff; pour the mixture evenly over the top of the bars.

Return to the oven and bake for an additional 15-20 minutes or until the center appears set and the top is golden and bubbly. Cool the bars COMPLETELY in the pan before cutting into squares. I recommend letting them stand overnight so the marshmallow fluff has a chance to set up completely. Serve! Store leftovers airtight at room temperature for several days.