Preheat an oven to 350°F (180°C) and line an 8x8 inch pan or with parchment paper. Leave an overhang of paper for easy removal.

In a saucepan or a microwave-safe bowl, melt butter and chopped chocolate. Heat it on low heat and stir with a wooden spoon or rubber spatula until it's all melted and combined. Alternatively, place it in the microwave and melt it together in 20-second intervals, stirring each time.

Once it's all melted, stir through the sugars while the mixture is still hot.

Beat in the eggs, one at a time, for around 1 minute until well combined, followed by the salt, vanilla extract and peppermint extract. 

Sift cocoa powder and stir it in well. Finally, add the flour to the chocolate mixture and whisk it well to combine it into a glossy chocolate batter. Pour the brownie batter into the prepared baking pan.

The baking time will depend on your oven or square baking pan used. Take care not to overcook them or the texture of the brownies won't be fudgy.

Leave the baked brownies in the pan to cool down completely on a wire rack.

In the bowl of a stand mixer fitted with a paddle attachment, or with an electric hand mixer, beat the butter until light, smooth and creamy.

Add in the powdered sugar, peppermint extract and heavy cream. Whip on low speed to combine the sugar, then beat on high speed for 3-4 minutes until, thick and fluffy. Stir through crushed candy pieces.

Once the brownie has completely cooled, add on the frosting on top of the brownies and use an offset spatula to smooth it out. Top with additional candy cane pieces.

Place the brownies in the refrigerator for around 20 minutes to slightly firm up the buttercream. For extra clean cuts, let it chill longer. Then remove the brownies from the pan, and slice into 16 pieces.