Preheat oven to 350*F and grease an 8-inch baking pan.

Bring a saucepan half full of water to a gentle simmer. Place the unsweetened and semisweet chocolates and butter into a small heatproof bowl. Set over simmering water. Stir until melted and smooth. Let cool briefly.

Ina bowl, sift together flour and baking soda, set aside. In a separate bowl, combine brown and granulated sugars and eggs. Using an electric mixer or wire whisk, beat until light and thick consistency, 7-8 minutes with the whisk or 2-3 minutes with a mixer. Stir in 1 teaspoon of peppermint extract. Then, stir into cooled chocolate mixture. Add in flour mixture and mint chocolate chips, mixing until just incorporated. Pour into prepped pan, and bake until top looks dry, about 30 minutes.

Meanwhile, put the confectioners sugar into a bowl and add milk until it forms a paste with a consistency of heavy cream. Stir in remaining 1 1/2 teaspoons of peppermint extract and a few drops of food coloring, if using.

When you take out the brownie cake, let it cool for 10 minutes. Then spread frosting onto it and even out with a metal spatula and let cool. Cut into 2-inch squares and store into covered containers at room temperature for 1 week