01 - Preheat oven to 350°F. Line an 8-inch square baking pan with parchment paper, leaving an overhang for easy removal.
02 - In a microwave-safe bowl, melt the butter. Whisk in sugar, then eggs and vanilla, mixing until smooth.
03 - Sift in flour, cocoa powder, and salt. Fold together until just combined. Stir in chocolate chips.
04 - Spread batter evenly in the prepared pan. Bake for 25-30 minutes, until a toothpick comes out with moist crumbs. Cool completely in the pan.
05 - Beat softened butter with powdered sugar until smooth. Add milk, peppermint extract, and a drop of green food coloring if desired. Beat until creamy and spreadable.
06 - Spread the mint mixture evenly over the cooled brownies. Refrigerate for 20 minutes to set.
07 - Heat heavy cream until steaming (not boiling). Pour over chocolate chips in a heatproof bowl. Let sit 1 minute, then stir until smooth.
08 - Pour ganache over the mint layer, spreading evenly. Chill brownies for at least 30 minutes before slicing.