Preheat oven to 350°F and grease or line an 8x8-inch baking dish.
Prepare the brownie mix in a large bowl according to package directions using water, oil, and egg.
Stir in the peppermint extract, Andes mint chips, and chopped Thin Mint cookies.
Pour the batter into the prepared baking dish and spread evenly.
Bake for 35–40 minutes, or until a toothpick inserted comes out with a few moist crumbs.
Let the brownies cool completely before cutting and serving.
Optional: Sprinkle with additional Andes mints and dark chocolate chips while warm.