Preheat the oven to 325°F. Line an 8-inch baking pan with parchment paper, extending the paper 2 inches over the sides to act as handles for easy lifting after the brownies have baked.

Stir together the flour, cocoa powder, baking powder, and salt in a medium bowl. Stir together the sugar, mayonnaise, eggs, and vanilla in a large bowl. Add the flour mixture to the sugar mixture, stirring until combined. Stir in the melted chocolate.

Bake the brownies at 325°F for 40 minutes, or until a toothpick inserted 1 inch from an edge comes out clean. Remove to a wire rack to cool completely.

Prepare the Peppermint Frosting: Beat the butter at medium speed with an electric mixer until creamy. Add the powdered sugar and beat until fluffy. Add 1 teaspoon milk and beat until the mixture is smooth; add more milk if needed. Beat in the peppermint extract.

Prepare the Chocolate Drizzle: Place the butter and chocolate chips in a microwave-safe dish. Microwave at HIGH in 30-second intervals until the butter and chocolate are melted. Stir until smooth. Let cool until the chocolateis drizzling consistency.

Use the parchment handles to lift the brownies from the pan. Remove the parchment paper. Spread the Peppermint Frosting evenly over the brownies. Spoon the Chocolate Drizzle over the frosting, and sprinkle with crushed peppermints. Let stand until the chocolate hardens (or refrigerate for 15 minutes). Cut into 16 squares.