Empty one sleeve of thin mints into a Ziploc bag. Crush cookies into smaller pieces using a rolling pin or wooden spoon. Set aside.
In a medium sized bowl, combine brownie mix, egg, vegetable oil and milk. Beat until thoroughly mixed.
Line a 9×13” pan with parchment paper or a baking mat. Pour brownie mixture over the baking mat and spread evenly. Empty crushed cookies onto the top of the brownie mixture and spread evenly.
For the cookie
Soften butter in the microwave 10-15 seconds. Combine butter with both light brown sugar and granulated sugar. Mix until smooth and mixture becomes fluffy.
Add egg and vanilla extract and beat until egg is thoroughly incorporated.
In a separate bowl, combine flour, salt and baking soda. Slowly add flour mixture to dough batter while mixing at a low speed. Once all the flour has been added, increase speed to medium until dough forms. Add mint chocolate chips last and fold into cookie dough.
Take the cookie dough and drop chunks onto the top of the brownie mixture, gently pressing them down into brownie batter.
Bake for 23-25 minutes at 350°F. Remove from oven and allow to cool before cutting. Since I used a baking mat, I was able to completely remove the sheet of brookies from the pan before cutting.