Instructions

• Preheat oven to 350°F. Lightly spray a 9" x 13" baking pan with non-stick spray and set aside.
• Brownie base — Melt chocolate and butter: In a medium saucepan over medium heat, combine 4 ounces unsweetened chocolate and 2/3 cup unsalted butter. Stir regularly until fully melted and smooth, taking care not to scorch. Remove from heat.
• Add sugar, eggs, and vanilla: Stir 2 cups packed light brown sugar, 3 large eggs, and 1 teaspoon pure vanilla extract into the melted chocolate until well combined and smooth.
• Add dry ingredients: Stir in 1 1/4 cups all-purpose flour, 1 teaspoon baking powder, and 1 teaspoon kosher salt until just combined and no large streaks of flour remain. Do not overmix.
• Bake: Spread the batter evenly in the prepared pan. Bake about 22 minutes, or until a toothpick inserted near the center comes out with just a few moist crumbs. For fudgy brownies avoid overbaking.
• Cool completely: Remove pan from oven and let the brownie base cool completely in the pan on a wire rack before frosting.
• Mint buttercream — Combine ingredients: In the bowl of a stand mixer, on low speed, mix 2 1/2 cups powdered sugar, 2 1/2 tablespoons heavy cream, 10 tablespoons softened unsalted butter, and 1 teaspoon pure mint extract until incorporated.
• Whip buttercream: Increase mixer to medium-high and beat for 3 minutes, until the frosting is light and fluffy. Tint with green food coloring if desired. If the buttercream is too soft to spread, chill briefly until spreadable.
• Frost and chill: Spread the mint buttercream in an even layer over the completely cooled brownie base. Place the pan in the freezer for 20 minutes to firm the buttercream.
• Chocolate glaze — Melt chocolate and butter: In a small saucepan over low heat, melt 5 ounces semi-sweet chocolate and 5 tablespoons unsalted butter, stirring until smooth. Remove from heat.
• Glaze the brownies: Remove the pan from the freezer. While the glaze is warm (not piping hot), quickly and gently spread it over the cold buttercream in an even layer.
• Set and cut: Let the glaze cool and harden at room temperature, or refrigerate 5–10 minutes to speed up setting. Once the glaze is firm, cut into squares and serve.