Step 1

Adjust oven rack to middle position and heat oven to 350 degrees. Line 8-inch baking dish with foil and coat foil with cooking spray.

Step 2

Melt chocolate and butter in small bowl in microwave or in heatproof bowl set over pan of simmering water, stirring occasionally to combine. Cool mixture for several minutes.

Step 3

Whisk sugar, baking powder, salt, eggs, and vanilla together in medium bowl until combined, about 15 seconds. Whisk in chocolate mixture until smooth. Stir in flour until no streaks of flour remain. Scrape batter evenly into prepared pan.

Step 4

Bake until toothpick inserted halfway between edge and center of pan comes out clean, 22 to 27 minutes.

Step 5

For the frosting: Remove brownies from oven, sprinkle with white chocolate chips, and let sit until the chips have softended but not melted, about 5 minutes. Using spatula or table knife, smooth chocolate evenly over top of brownies. Sprinkle with ground peppermint candies. Cool brownies on wire rack until topping firms up, about 2 hours.

Step 6

Using foil, lift brownies from pan to cutting board. Slide foil out from under brownies and cut brownies into 2-inch squares. (Brownies can be stored in airtight container for several days.)