Preheat oven to 350º F. Generously butter a 9-inch square metal baking pan. Add a layer of parchment paper, leaving the edges long enough to lift the brownies out of the pan for easier slicing. Butter the parchment paper, pressing it down to help it adhere smoothly to the pan. Set aside.
To prepare the brownies, melt chocolate and butter together in a double boiler or in short bursts in the microwave. Stir until well combined. Set aside.
In a medium-sized mixing bowl, whisk together sugar, salt, and eggs. Add chocolate mixture, mint extract, and chocolate chips, whisking until well combined. Gently stir in flour, taking care not to over mix.
Pour the batter into the prepared pan and bake for 25 minutes. Remove from the oven and cool to room temperature.
To prepare the frosting, combine confectioners' sugar, melted butter, milk, and mint extract in a medium-sized bowl. Whisk until smooth. Spread the frosting evenly over the cooled brownies.
For the glaze, melt chocolate and butter together in a double boiler or in the microwave. Mix well to combine. Drizzle glaze over the frosted brownies. If the glaze is too thick to drizzle, gently reheat.
Refrigerate to set the glaze. Lift the brownies from pan using the parchment paper and slice into 1½-inch squares. Serve chilled or at room temperature.