01 - Preheat oven to 350°F. Line an 8x8-inch baking pan with parchment paper, leaving an overhang for easy removal.
02 - In a heatproof bowl, melt butter and chocolate chips together over a pot of simmering water or in the microwave in 30-second bursts, stirring until smooth. Cool slightly.
03 - Whisk in sugar, then eggs one at a time, and vanilla extract. Fold in flour and salt just until combined.
04 - Spread batter evenly in prepared pan. Bake for 25-30 minutes or until a toothpick inserted comes out with a few moist crumbs. Cool completely in the pan.
05 - Beat softened butter until fluffy. Gradually add powdered sugar, milk, and peppermint extract. Mix until smooth. Add green food coloring to desired shade.
06 - Spread mint cream evenly over cooled brownies. Chill in the refrigerator for at least 30 minutes.
07 - Heat heavy cream until steaming (not boiling). Pour over chocolate chips and butter in a bowl. Let stand 2 minutes, then stir until smooth.
08 - Pour ganache over chilled mint layer and spread evenly. Chill again until set, about 30-45 minutes.
09 - Lift brownies from pan using parchment overhang. Cut into 16 squares with a sharp, warm knife for clean edges.