Grease insert of 4.5 QT Crockpot™ with cooking spray. Line bottom of insert with enough parchment paper to extend 4 inches up side; set aside. Stir together flour, cocoa powder, baking powder and salt; set aside.
In heatproof bowl set over saucepan of barely simmering water, melt unsweetened chocolate, stirring occasionally. Remove from heat; stir in sugar. Let cool slightly.
Stir in eggs and egg yolk, one at a time, until well combined. Stir in vanilla. Stir in flour mixture.
Scrape half of the batter into prepared Crockpot™; smooth top. Top with peppermint patties. Top with remaining batter. Cover and cook on Low for 3 ½ hours. Uncover and cook for about 30 minutes or until edge is completely set and just a few moist crumbs adhere to toothpick when inserted into center of brownie.
Carefully remove brownie using parchment paper. Turn out onto rack. Let cool completely. Cut into 10 pieces.
Tip: For extra-minty brownies, beat together 1 cup heavy cream, ¼ cup icing sugar and ½ tsp mint extract. Dollop over brownies and drizzle with chocolate sauce.