01 - Preheat oven to 350°F. Line an 8-inch square baking pan with parchment paper.
02 - In a mixing bowl, melt the butter. Whisk in sugar, eggs, and vanilla until combined.
03 - Sift in cocoa powder, flour, and salt. Mix until just combined.
04 - Fold in chocolate chips. Pour batter into prepared pan and spread evenly.
05 - Bake 25-30 minutes, or until a toothpick inserted in the center comes out with a few moist crumbs. Cool completely.
06 - Beat softened butter until creamy. Gradually add powdered sugar, milk, peppermint extract, and a few drops of green food coloring (if using); mix until smooth and spreadable.
07 - Spread the mint layer evenly over the cooled brownies. Refrigerate 20 minutes to set.
08 - Melt chocolate chips and butter together in the microwave or double boiler, stirring until smooth.
09 - Pour the chocolate topping over the mint layer. Spread evenly and refrigerate until set, about 30 minutes.
10 - Cut into squares and serve chilled or at room temperature.