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.
Line the bottom and sides of a 9×13 baking pan with aluminum foil or 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.
Once completely cooled, lift the foil out of the pan using the overhang on the sides. Place the whole thing on a baking sheet as you make the frosting. Do not cut into squares yet.
Mint Frosting Layer
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.
Taste and add a drop or two more peppermint extract if desired.
Frost cooled brownies, that you placed on the baking sheet, and place the baking sheet in the refrigerator.
Keep 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. Or melt in a medium microwave safe bowl in 20 second increments, stirring after each, in the microwave.
Once melted and smooth, pour over mint layer. Gently spread with a knife or offset spatula.
Place the brownies, that are still on the baking sheet, 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.