Preheat oven to 375F. Lightly grease an 8x8 inch baking pan with non-stick cooking spray. Then line with parchment paper so that it is easy to lift the brownies out of pan once baked. Set aside.
In a medium bowl, whisk flour, salt, baking powder, and cocoa powder. Set aside.
In another medium bowl, combine butter, semi-sweet chocolate chips and dark chocolate wafer. Melt in the microwave over medium heat, stirring occasionally, until smooth. Set aside to cool slightly.
In a large bowl, beat sugar and eggs until light and fluffy. Beat in vanilla extract. Stir in melted chocolate. Slowly stir in flour mixture until just incorporated.
Spoon batter into prepared pan. Bake for about 30 minutes, or until toothpick inserted into middle comes out with just a few crumbs, and edges are set. Remove from oven and allow to cool completely.
To make the peppermint frosting, beat butter in a large bowl with an electric hand mixer or stand mixer with fitted paddle attachment until light and creamy, about 2-3 minutes. Add in powdered sugar, 1/2 cup at a time, until fully incorporated.
Add milk and peppermint extract. Beat for about 2-3 minutes, or until light and fluffy. Gently fold crushed candy canes into frosting, if desired, or reserve to sprinkle on top of frosted brownies.
Lift brownies out of pan by grabbing onto the parchment paper. Spoon frosting on top of brownies and spread evenly. Add crushed candy canes, if desired. Cut into squares and enjoy!