Preheat oven to 350 degrees F. Line a 13x9" pan with foil, extending the sides of the foil over the edges of the pan. Liberally grease 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 forms. Spread the mixture evenly into the prepared pan. Bake for approx. 12 minutes, then remove from the oven but keep the oven on.

Top the crust layer with the Hershey's bars to cover the entire surface. Immediately top with the assorted chocolate chips evenly. In a separate small bowl, whisk together the hot fudge and sweetened condensed milk; pour the mixture evenly over the bars.

Return to the oven and bake for 20-22 minutes or until the center is just about set and bubbly. Cool completely in the pan, preferably overnight. Once bars are completely cooled, cut into squares and serve.