01 - Preheat oven to 350°F. Line an 8-inch square baking pan with parchment paper, allowing paper to extend over sides for easy removal.
02 - Melt butter in microwave-safe bowl. Whisk in sugar until combined, then add eggs and vanilla, beating until smooth. Sift in flour, cocoa powder, and salt. Fold until just combined, then stir in chocolate chips.
03 - Spread batter evenly in prepared pan. Bake 25-30 minutes until toothpick inserted comes out with moist crumbs. Cool completely in pan on wire rack.
04 - Beat softened butter with powdered sugar until smooth and creamy. Add milk, peppermint extract, and food coloring if using. Continue beating until light and spreadable.
05 - Spread mint mixture evenly over cooled brownies. Refrigerate 20 minutes until set.
06 - Heat heavy cream until steaming but not boiling. Pour over chocolate chips in heatproof bowl. Let stand 1 minute, then stir until completely smooth and glossy.
07 - Pour ganache over mint layer, spreading evenly. Refrigerate at least 30 minutes until ganache is set. Lift from pan using parchment overhang and cut into squares.