1 cup (230g) unsalted butter
8 ounces semi-sweet chocolate, coarsely chopped
1 and 1/2 cups (300g) granulated sugar
1/2 cup (100g) packed light brown sugar
4 large eggs, at room temperature
2 teaspoons vanilla extract
1/2 teaspoon salt
1/2 cup + 3 Tablespoons (85g) all-purpose flour (spoon & leveled)
1/4 cup (21g) natural unsweetened cocoa powder
MINT FROSTING LAYER
1/2 cup (115g) unsalted butter, softened to room temperature
2 cups (240g) confectioners' sugar
2 Tablespoons (30ml) milk
1 and 1/4 teaspoons peppermint extract1
1 drop liquid or gel green food coloring (optional)
1/2 cup (115g) unsalted butter
1 heaping cup (around 200g) semi-sweet chocolate chips
For the brownies: 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 350º. Line the bottom and sides of a 9x13 baking pan1 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. 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.
For the 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. This allows the frosting to "set" on top of the brownies which makes spreading the chocolate layer easy. Keep in the refrigerator for at least 1 hour and up to 4 hours.
For the chocolate layer: 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.