01 -
Melt butter and chopped chocolate in a medium saucepan over medium heat, stirring constantly for about 5 minutes, or in a microwave-safe bowl in 20-second increments. Transfer to a large mixing bowl and allow to cool for 10 minutes.
02 -
Position oven rack in lower third position and preheat to 177°C. Line a 9×13-inch baking pan with parchment paper, leaving overhang on all sides for easy removal later.
03 -
Whisk granulated and brown sugars into the cooled chocolate mixture. Add eggs one at a time, whisking until smooth after each addition. Incorporate vanilla, then gently fold in salt, flour, and cocoa powder.
04 -
Pour batter into prepared pan and bake for 35-36 minutes until brownies begin to pull away from edges. A toothpick inserted should come out with just a few moist crumbs. Cool completely in the pan on a wire rack.
05 -
Beat butter until smooth and creamy, about 2 minutes. Add confectioners' sugar and milk, beating on low for 2 minutes, then on high for 1 minute. Add peppermint extract and food coloring (if using) and beat on high for 1 minute.
06 -
Spread mint frosting evenly over cooled brownies. Refrigerate for at least 1 hour to set the frosting before adding the chocolate layer.
07 -
Melt butter and chocolate chips in a saucepan over medium heat or in microwave in 20-second increments. Pour over set mint layer and spread evenly with an offset spatula.
08 -
Refrigerate for at least 1 hour to set the chocolate. Cut into squares using a sharp knife, wiping the blade clean between cuts for neat edges.