Preheat oven to 350 degrees F. and line a 9x13 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.

After 5 minutes of cooling, run a plastic knife around edges to loosen, this makes it much easier to remove from foil.  Let cool completely, remove foil from pan, then cut into squares.  I cut off all of the edges before cutting into squares (makes for neater squares), but do whatever you'd like!!