Preheat the oven to 325° and line an 8x8 inch baking pan with parchment paper and set aside.
In a large bowl, mix together your brownie mix, egg, water and oil until well combined. Add in 3/4 of the crushed Thin Mint cookies and mix. Pour mixture into the prepared pan and spread the batter out evenly. Top the brownie mix with the remaining cookie pieces.
Bake in oven for about 40-45 minutes or until the brownies are set and a toothpick comes out clean. Let cool completely then enjoy!