Preheat oven to 350°F. Line a 9-inch square baking dish with a parchment paper sling or spray with baking spray.
In a large microwave-safe bowl, combine the bittersweet chocolate and butter. Microwave on high in 20-30 second bursts, stirring well between bursts of heat until melted and smooth. Whisk in the cocoa powder.
¾ cup (170g) salted butter, 4 ounces bittersweet chocolate, ½ cup (43g) unsweetened cocoa powder
Add the granulated sugar, whisking well to combine. Add the eggs and vanilla extract, beating well either by hand with a whisk or using a hand mixer.
1 cup (200g) granulated sugar, 2 teaspoons pure vanilla extract, 3 large eggs
Sprinkle the flour and salt over the brownie batter and gently stir it in with a wooden spoon or spatula. Stir in the chocolate chips.
¾ cup (95g) all-purpose flour, ½ teaspoon salt, ¾ cup semisweet chocolate chips
Pour half of the brownie batter into the prepared baking dish and spread into the corners. Arrange most of the whole peppermint patties in an even layer on top of the brownie batter, reserving about 4 to chop into quarters. Top with the remaining brownie batter, spreading it into an even layer. Sprinkle the additional chopped York peppermint patty pieces on top of the brownies.
1 (10.1-ounce) bag fun-size York peppermint patties
Bake for 30 to 35 minutes, until a toothpick inserted into the center of the brownies comes out mostly clean with only a few crumbs attached. Be sure not to overbake.
Cool completely before cutting.