01 - Preheat 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 granulated and brown sugars until thoroughly combined.
03 - Add eggs one at a time, mixing well after each addition. Stir in vanilla extract until smooth.
04 - Sift together flour, cocoa powder, salt, and baking powder. Fold into the wet mixture until just combined.
05 - Spread batter evenly in prepared pan. Bake 28–32 minutes, until a toothpick inserted in center returns with a few moist crumbs. Cool completely in pan.
06 - Beat softened butter until smooth. Gradually add powdered sugar and milk, beating until light and fluffy. Mix in peppermint extract and green food coloring to desired shade.
07 - Spread mint frosting evenly over cooled brownies. Refrigerate 20 minutes to set.
08 - Combine chocolate chips, heavy cream, and butter in microwave-safe bowl. Microwave in 20-second bursts, stirring between, until smooth and glossy.
09 - Pour ganache over mint layer, spreading evenly. Chill at least 30 minutes until set.
10 - Lift brownies from pan using parchment overhang. Cut into squares and serve.