01 - Preheat the oven to 350°F (175°C). Line a 9x9-inch baking pan with parchment paper, allowing excess to overhang for easy removal.
02 - In a heatproof bowl set over a saucepan of simmering water (creating a double boiler), melt the unsalted butter and semisweet chocolate designated for the brownie layer. Stir continuously until the mixture is smooth and fully incorporated. Remove from heat and allow to cool slightly.
03 - Gradually whisk the granulated sugar into the cooled chocolate mixture. Incorporate the large eggs one at a time, ensuring each egg is fully combined before adding the next. Stir in the vanilla extract.
04 - Gently fold in the all-purpose flour and salt until just combined. Exercise caution not to overmix the batter to maintain a tender crumb.
05 - Pour the brownie batter into the prepared 9x9-inch baking pan and spread evenly. Bake for 30–35 minutes, or until a wooden skewer inserted into the center emerges with moist crumbs attached. Allow the brownies to cool completely within the pan on a wire rack.
06 - For the mint frosting, cream the softened unsalted butter in a mixing bowl until light and fluffy. Gradually incorporate the powdered sugar and whole milk, beating until the frosting is smooth and aerated. Stir in the pure peppermint extract and a few drops of green food coloring until the desired vibrant green hue is achieved.
07 - Carefully spread the prepared mint frosting evenly over the completely cooled brownie layer. Transfer the pan to the refrigerator and chill for 20 minutes to allow the frosting to set firmly.
08 - For the chocolate ganache, gently heat the heavy cream in a small saucepan until it just begins to simmer. Pour the hot cream over the chopped semisweet chocolate and unsalted butter in a heatproof bowl. Let the mixture stand undisturbed for 2 minutes, then stir vigorously until the ganache is smooth, glossy, and fully emulsified.
09 - Carefully pour the warm chocolate ganache over the chilled mint frosting layer, spreading it evenly to cover the entire surface. Return the pan to the refrigerator and chill for at least 30 minutes, or until the ganache is completely set.
10 - Using the parchment paper overhang, carefully lift the brownies from the baking pan. Place on a cutting board and slice into 16 uniform squares. Serve chilled or at room temperature, as preferred.