Preheat oven to 350º F. Prepare brownie mix per box instructions. Line a 13x9" pan with foil (you may also use a 15x10" jelly roll pan--see note for directions) and lightly grease. Spread brownie mix in pan evenly and place in oven. Bake 25 minutes and begin checking for doneness--remove brownie layer from the oven when a toothpick inserted into the center of the brownies comes out almost clean. Cool on a baking rack.

Meanwhile, make the frosting layer. In the bowl of a stand mixer or in a medium mixing bowl with tall sides, combine 7 tablespoons butter, melted, with confectioners' sugar, heavy cream, and 1 teaspoon peppermint extract. Beat on medium speed until smooth. When brownies have cooled spread this layer evenly over the brownies. I used a piece of wax paper to help smooth out this layer evenly. Refrigerate for 30 minutes.

In a microwave-safe bowl, combine semi-sweet chocolate and 1/2 cup butter. Microwave in 30-second bursts, stirring well in between, until chocolate is totally melted and smooth. Stir in 1 teaspoon peppermint extract and spoon onto chilled frosting layer. Use a spatula to smooth the ganache layer evenly over and sprinkle with crushed candy canes or peppermint candies. Chill until ganache layer is hardened. Use the sides of the foil to lift entire batch out of pan and cut into individual squares. A sharp knife, warmed under hot running water, will slice through the hardened ganache layer beautifully.