Preheat oven to 350F (175C) and prepare an 8x8 baking pan by lining with parchment paper or aluminum foil. Set aside.
Combine butter pieces and chopped chocolate in a large microwave-safe bowl.
Microwave for 30 seconds. Stir well, return to microwave and continue to heat at 15-second increments (stirring between) until chocolate and butter are completely melted.
Add cocoa powder and sugar, stir well.
Add eggs, one at a time, stirring well after each addition.
Stir in vanilla extract. Sprinkle salt over batter and stir to combine.
Add flour, stirring until completely combined.
Spread half of the brownie batter into prepared pan.
Take your unwrapped OREO Mint Chocolate Candy Bars and press them firmly into the brownie batter spacing evenly to completely cover the batter (you’ll need to break several candy bars in half, you will use a total of 7 ½ candy bars -- see photo. Reserve remaining 2.5 candy bars for topping).
Spread remaining brownie batter evenly over candy bars so that they are completely covered. Transfer pan to oven and bake on 350F (175C) for 45-55 minutes or until a toothpick inserted in the center comes out with fudgy crumbs (or clean).
Allow brownies to cool completely before covering Ganache.
To prepare ganache, combine chocolate chips, heavy cream, and butter in a small saucepan over medium-low heat.
Stir frequently until chocolate and butter are completely melted and ingredients are well-combined.
Allow ganache to cool for 10 minutes before pouring over brownies. While the ganache cools, chop up your remaining 2 ½ OREO Mint flavored Chocolate Candy Bars.
Pour ganache over cooled brownies.
Sprinkle chopped candy pieces over ganache. Allow ganache to set before cutting and serving (you can expedite this by placing brownies in the refrigerator).