01 - Preheat the oven to 350°F. Line a 9x13-inch baking pan with parchment paper, leaving an overhang for easy removal.
02 - In a large bowl, whisk melted butter, granulated sugar, and brown sugar until combined. Beat in eggs one at a time, then add vanilla.
03 - Sift in flour, cocoa powder, and salt. Stir until just combined. Fold in chocolate chips.
04 - Spread the brownie batter evenly into the prepared pan. Bake for 30-35 minutes, or until a toothpick inserted in the center comes out with a few moist crumbs. Cool completely in the pan.
05 - Beat softened butter until creamy. Gradually add powdered sugar, milk, peppermint extract, and a few drops of green food coloring (if using). Beat until smooth and spreadable.
06 - Spread the mint layer evenly over the cooled brownies. Refrigerate for 20 minutes.
07 - Melt chocolate chips and butter together in a microwave or double boiler until smooth. Cool slightly, then pour over the mint layer. Tilt the pan to cover evenly.
08 - Refrigerate for 30 minutes or until set. Lift brownies out using the parchment overhang and cut into squares.