Preheat oven to 350 degrees F. and line a 9x13 inch baking dish with non stick cooking spray.
Place cake mix, butter and egg into a large bowl, mix until dough forms. Using hands is easiest to mix together. Press dough into bottom of prepared baking dish. Top evenly with chocolate chips and Andes mints. Drizzle sweet milk over top and top with broken pieces of Thin Mint Cookies.
Bake for 30-35 minutes, until baked through. Immediately after removing from oven, take a plastic knife around edges to loosen. Let cool completely. Cut into squares and serve.