Preheat oven to 350 degrees (F).
Cream butter and sugar with an electric mixer on medium speed, until light and fluffy, about 3 minutes.
Beat in the cocoa powder until thoroughly incorporated and no lumps remain.
Beat in eggs, one at a time, then add salt and peppermint extract. Mix well.
Gradually add in the flour and stir until completed incorporated.
Line a jelly roll pan (15-1/2"x10-1/2"x1") with parchment paper. Spritz the parchment paper with non-stick cooking spray, then spread the batter evenly in the prepared pan.
Bake for about 30 minutes or until a wooden skewer inserted in the center comes out mostly clean. Rest pan on a wire rack until completely cool before proceeding with frosting.
Cream the butter then slowly add the confectioners' sugar and beat until incorporated completely.
Add the peppermint extract.
Mix in 4 tablespoons heavy whipping cream. Add additional cream, up to 1 tablespoon, until spreadable consistency is reached. Whip for an additional 3 minutes, until frosting is light and fluffy.
Frost the brownies then cut into 2-inch squares. Garnish each square with bits of broken candy canes right before serving.