Melt the butter and chopped chocolate in a medium saucepan on medium heat, stirring constantly, about 5 minutes. Or melt in a medium microwave safe bowl in 20 second increments, stirring after each, in the microwave. Remove from the heat, pour into a large mixing bowl, and allow to slightly cool for 10 minutes.

Adjust the oven rack to the lower third position and preheat oven to 350°F (177°C). Line the bottom and sides of a 9×13-inch baking pan with parchment paper, leaving an overhang on all sides. Set aside.

Whisk the granulated and brown sugars into the cooled chocolate/butter mixture. Add the eggs, one at a time, whisking until smooth after each addition. Whisk in the vanilla. Gently fold in the salt, flour, and cocoa powder. Pour batter into the prepared baking pan and bake for 35-36 minutes or until the brownies begin to pull away from the edges of the pan. A toothpick inserted in the center should come out with only a few moist crumbs when the brownies are done. Allow the brownies to cool completely in the pan set on a wire rack.

In a medium bowl using a handheld or stand mixer fitted with a paddle attachment, beat the butter on medium speed until smooth and creamy, about 2 minutes. Add the confectioners’ sugar and milk. Beat for 2 minutes on low speed, then increase to high speed and beat for 1 additional minute. Add the peppermint extract and food coloring (if using) and beat on high for 1 full minute. Frost cooled brownies, and place the baking sheet in the refrigerator for at least 1 hour and up to 4 hours.

Melt the butter and chocolate chips in a medium saucepan on medium heat, stirring constantly, about 5 minutes. Once melted and smooth, pour over mint layer. Gently spread with a knife or offset spatula. Place the brownies in the refrigerator and chill for 1 hour (and up to 4 hours or even overnight) to set the chocolate.

Once chilled, remove from the refrigerator and cut into squares. For neat cutting, make very quick cuts, using a very sharp large knife, and wiping the knife off with a paper towel between each cut. Brownies are OK at room temperature for a few hours. Cover tightly and keep leftovers in the refrigerator for up to 5 days.