01 - Preheat the oven to 350°F. Line a 9x9-inch baking pan with parchment paper, leaving an overhang for easy removal.
02 - In a microwave-safe bowl, melt the butter. Whisk in both sugars until well combined. Add eggs one at a time, mixing well after each addition. Stir in vanilla.
03 - Sift together flour, cocoa powder, salt, and baking powder. Fold the dry ingredients into the wet mixture until just combined.
04 - Spread the brownie batter evenly into the prepared pan. Bake for 28-32 minutes, or until a toothpick inserted in the center comes out with a few moist crumbs. Let cool completely in the pan.
05 - In a medium bowl, beat the softened butter until smooth. Gradually add powdered sugar and milk, beating until light and fluffy. Mix in peppermint extract and green food coloring until desired color is reached.
06 - Spread the mint frosting evenly over the cooled brownies. Refrigerate for 20 minutes to set.
07 - In a microwave-safe bowl, combine chocolate chips, heavy cream, and butter. Microwave in 20-second bursts, stirring in between, until smooth and glossy.
08 - Pour the ganache over the mint layer, spreading evenly. Chill for at least 30 minutes until set. Lift the brownies from the pan using the parchment overhang. Cut into squares and serve.