01 - Preheat oven to 350°F. Line an 8-inch square baking pan with parchment paper, ensuring paper extends over edges for easy removal.
02 - Melt butter in a mixing bowl. Whisk in sugar, eggs, and vanilla until thoroughly combined and smooth.
03 - Sift cocoa powder, flour, and salt into the wet mixture. Mix until just combined, being careful not to overmix.
04 - Fold semisweet chocolate chips into the batter. Pour into prepared pan and spread evenly with a spatula.
05 - Bake 25–30 minutes until a toothpick inserted in the center comes out with a few moist crumbs. Cool completely in the pan before proceeding.
06 - Beat softened butter until creamy. Gradually add powdered sugar, milk, peppermint extract, and green food coloring if desired. Mix until smooth and spreadable.
07 - Spread mint filling evenly over cooled brownies. Refrigerate for 20 minutes to set the layer.
08 - Melt chocolate chips and butter together using microwave or double boiler, stirring frequently until completely smooth and glossy.
09 - Pour chocolate topping over mint layer and spread evenly. Refrigerate until fully set, approximately 30 minutes. Cut into squares before serving.