Step 1Heat the oven to 350 degrees. Lightly coat a half-sheet pan (17-by-13-by-1-inch) with nonstick cooking spray. Line the bottom and sides with parchment paper and spray the parchment.
Step 2Combine the chocolate chips and butter in a large sauce pot. Melt over medium-low heat, stirring occasionally to prevent scorching, until smooth. Remove the pot from the heat and let cool slightly. In a medium bowl, whisk together the sugar, flour, baking powder and salt.
Step 3Add the eggs to the melted chocolate one at a time, whisking well after each addition. Add the dry ingredients and stir with the whisk just until blended. Pour into the prepared pan and spread evenly.
Step 4Bake until a toothpick inserted 2 inches from the edge comes out clean, 20 to 22 minutes. Transfer to a wire rack. Let cool for 2 minutes, then gently lay the peppermint bark on top in a single layer. Cool completely in the pan on the rack. Use the parchment paper to slide the brownies out of the pan and cut into 2- to 3-inch squares.
Double Mint Brownies: Add ½ teaspoon peppermint extract to the brownie batter after mixing in the dry ingredients.