01 - Preheat oven to 350°F. Line an 8x8 inch baking pan with parchment paper, ensuring overhang on sides for easy removal.
02 - Whisk melted butter and sugar in a bowl until combined. Add eggs and vanilla, mixing until smooth. Sift in flour, cocoa powder, and salt; fold gently until just combined.
03 - Pour batter into prepared pan. Bake for 25-30 minutes until a toothpick inserted in the center comes out with moist crumbs. Cool completely in the pan.
04 - Beat softened butter with powdered sugar until fluffy. Add milk, peppermint extract, and green food coloring. Beat until smooth and pale green. Spread evenly over cooled brownie base. Refrigerate for 20 minutes to set.
05 - Melt chocolate chips and butter together in a heatproof bowl over a pot of simmering water, stirring until smooth. Cool slightly, then pour over mint layer, spreading evenly. Chill for at least 20 minutes until set.
06 - Remove brownies from pan using parchment. Cut into 16 squares with a sharp knife, wiping the blade clean between cuts for neat edges.