Preheat oven to 350° and line a 9×13 inch baking pan with tin foil sprayed with non-stick cooking spray. Place cake mix, butter and egg into a large bowl, mixing to combine. (Use your hands, it works much quicker) Press into prepared baking pan. Top with broken pieces of cookies. Pour sweetened condensed milk over cookies and top with chocolate chips. Bake for 23-25 minutes, until cooked through. Remove from oven. Let cool completely, remove foil from pan, then cut into squares.
After 5 minutes of cooling, run a plastic knife around edges to loosen, this makes it much easier to remove from foil