Preheat oven to 350° and place rack in middle oven position. Line a 8X8 or 9X9 inch baking pan with parchment paper and spray with cooking spray or grease with butter.
In a heat proof bowl add the chopped bittersweet chocolate and the butter, chopped into tablespoon size pieces. Melt in 30 minute increments and stir between each until fully melted and smooth. Set aside. In a separate bowl add the eggs, egg yolks, granulated sugar and brown sugar. Whisk vigorously for about two minutes to incorporate air. Add the vanilla extract and the melted chocolate and butter mixture. Whisk together until combined for another minute.
In a separate bowl whisk together the flour, dutch process cocoa powder, salt, and baking powder. Add to the wet ingredients and fold together to combine. Pour into prepared baking dish. Press 16 sandwich cookies into the top of the brownie batter in a 4x4 pattern. Bake in preheated oven for 22-28 minutes. The top should look set and start to crack. A toothpick inserted into the middle may have a few moist crumbs but not be wet with batter when finished.
Once finished set aside to cool completely on a cooling rack. Leave brownies in the pan.
Mint Chocolate Ganache
Add chopped chocolate to a heat proof bowl. In a separate heat proof container microwave the heavy whipping cream for about a minute. It should feel like hot bath water. Pour over the chopped chocolate and set aside for two minutes to melt. Add in the peppermint extract. Stir until smooth and combined, this may take a minute or so. Pour over the brownies still in the pan and use a spatula to smooth towards the edges. Chop the remaining four mint sandwich cookies into pieces and sprinkle over the ganache while it is still wet. Place in the refrigerator for one hour to set up the ganache. Remove brownies from pan using parchment paper and slice into squares. Enjoy!
Notes
Store brownies in airtight container for two days. The cookies start to soften beyond that so I recommend eating them within the first day or two of baking.