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

In a large bowl, combine the cake mix, egg and oil with a spoon until a soft dough comes together. Press evenly into the prepared pan. Set aside.

In a small bowl, whisk together the marshmallow creme, peanut butter and sweetened condensed milk until combined. Mixture will be thick! Pour evenly over the top of the bar layer. Sprinkle the top evenly with the white chocolate chips.

Bake for approx. 23-25 minutes or until the edges are set and the top is a very light golden brown. Center may still be slightly jiggly, but do NOT over cook. You want the bars to be gooey and they'll continue to cook up as they cool. Allow the bars to cool completely before dusting with powdered sugar, if desired, and cutting into squares. Serve.