Preheat oven to 350. Prepare a 9 x 13" pan with cooking spray; set aside. In a mixing bowl, combine cake mix and condensed milk. Batter will be thick. Press 2/3 of batter into prepared pan. Bake for 8 minutes. Remove >From oven and pour caramel topping over cake. Sprinkle with chocolate chips. Crumble remaining batter over top. Bake for 17 minutes more or until top is dry to the touch. Cool.