Preheat oven to 350ºF. Line a 9" square pan with parchment paper, leaving an overhang.
Melt butter and chocolate in the microwave on 50% power for 2 minutes or until about 80% melted. Stir to finish melting. Stir in sugar and then eggs, one at a time, until well combined. Stir in mint extract and salt, then flour.
Spread into prepared pan and bake for about 25 minutes, or until edges are firm and centre is just set. Cool 5 minutes in pan, then lift by the parchment overhang and finish cooling completely on wire rack before frosting.
To make frosting, beat all ingredients together until light and fluffy. Start with 1/8 tsp mint extract and add up to 1/4 tsp to taste (I used 1/4 tsp and found it a teeny bit too strong, this extract is pretty potent, so better to err on the side of less to start). Frost the cooled brownies, then pop them in the freezer until you are ready to add ganache.
To make ganache, combine chocolate and cream in a microwave-safe bowl and microwave on 50% power for 2 minutes then stir to finish melting. Let cool for about 20 minutes at room temperature then pour over the chilled brownies, spreading to edges. Sprinkle with candy cane. Let sit at room temperature for 1 hour to set the ganache.
They'll keep at room temperature for several days, or they freeze really well.