Preheat the oven to 350°F (175°C) and prepare a 9x13 inch baking pan with parchment paper.
In a large mixing bowl, melt the unsalted butter and stir in the granulated sugar until combined.
Add the eggs, one at a time, mixing well after each addition. Then stir in vanilla extract.
Sift together the flour, cocoa powder, baking powder, and salt. Gradually add to the wet ingredients, mixing until combined.
Pour the brownie batter into the prepared pan, smoothing the top with a spatula.
Bake in the preheated oven for 25-30 minutes or until a toothpick inserted in the center comes out with a few moist crumbs.
Allow the brownies to cool in the pan for about 15 minutes before transferring them to a wire rack to cool completely.
While the brownies cool, prepare the mint frosting by whisking together the powdered sugar, cocoa powder, milk, and peppermint extract.
Spread the mint frosting evenly over the cooled brownies, and if desired, add a few drops of food coloring to enhance appearance.
Cut the brownies into squares and serve, optionally garnishing with additional mint leaves or chocolate shavings.