Preheat the oven to 350 degrees. Line a 9x9 baking dish with parchment paper or foil. Spray w/ non stick cooking spray if using foil.
Combine the brownie mix, butter, eggs and milk, mixing well.
Add in the 3/4 cup of Thin Mint cookie pieces.
Pour the brownie batter into the baking dish and spread out using a large spoon or spatula.
Sprinkle the extra cookie pieces on top of the brownie batter.
Bake for 35-40 minutes.
Remove from the oven and although the brownies to cool completely, prior to cutting.