Preheat oven to 350°F.
Melt unsalted butter and chocolate in a microwave-safe bowl in 30-second intervals, stirring until smooth.
Stir in granulated sugar until well combined.
Add eggs, one at a time, mixing well after each addition. Stir in pure vanilla extract.
Gently stir in flour and baking powder until just combined.
Grease a 9x13-inch baking pan or line it with parchment paper. Pour the batter into the prepared pan and bake for 25 to 30 minutes, or until a toothpick inserted in the center comes out clean.
Allow the brownies to cool completely in the pan on a wire rack.
In a bowl, beat butter until creamy. Gradually add powdered sugar, milk, and peppermint extract. Mix until smooth. Optionally, add green food coloring drops.
Spread the mint frosting evenly over the cooled brownies.
Refrigerate the brownies for about 30 minutes to set the frosting.
Melt semi-sweet chocolate chips and unsalted butter in a microwave-safe bowl in 30-second intervals, stirring until smooth.
Pour the chocolate mixture over the chilled mint layer and spread evenly.
Return the brownies to the refrigerator for about 1 hour, or until the chocolate topping is set.
Allow the brownies to rest at room temperature for 10 to 15 minutes before slicing them into squares. Serve and enjoy!