Grease an 8" square baking pan with vegetable oil spray. Cut an 8” by 12” piece of parchment paper to fit the bottom and up 2 of the 4 sides of the baking pan. (The parchment paper extending the sides of the pan will later be used as handles to lift the bar out of the pan once baked.) Preheat oven to 350°F.
Sift the flour, cocoa and baking powder into a bowl. Mix in the nuts. Place the sugar, butter and extract in a second bowl and beat to combine. Beat in the egg. Mix dry ingredients into the wet until just combined. Spread the thick batter into the pan. Bake for 25 minutes, or until set and the brownie springs back when touched in the centre.
Cool brownie to room temperature, and then finish as described below.
For the topping and to finish:
Place the milk, ¼ cup butter, icing sugar and peppermint extract in a bowl and beat until well combined. Spread the mixture evenly over the brownie. Refrigerate bar until the topping is set, about 1 to 2 hours.
Place the chocolate and 2 Tbsp. butter in a heatproof bowl and set over barely simmering water. Heat and stir until the chocolate has melted and blended with the butter.
Cool slightly, and then very carefully and evenly spread on the bar, lifting up and swirling the chocolate around the pan if needed. Refrigerate the bar again until the chocolate mixture is set. Now cover bar and keep refrigerated until ready to serve.
To serve, run a knife around the outside of the square to loosen it from the pan. Grab onto the parchment and lift the square out and onto a cutting board. Now cut into 32, 2” x 1" bars.
Note: Peppermint extract is sold in small bottles in our baking supply aisle