Preheat your oven to 350°F. Coat a 9x13-inch baking pan with nonstick spray and place it aside until ready to use.

In a medium saucepan, melt the unsweetened chocolate and butter together over medium heat, stirring often to avoid scorching.

Once the mixture is completely smooth and glossy, remove the pan from the heat. Mix in the brown sugar, eggs, and vanilla, stirring until everything is well blended.

Carefully fold in the flour, baking powder, and salt, mixing only until the batter comes together. Spread the batter evenly into the prepared pan.

Bake for about 22 minutes, or until a toothpick inserted in the center comes out with only a few moist crumbs. This ensures a fudgy texture. Let the brownies cool fully in the pan before proceeding to the next step.

In a stand mixer bowl, blend together the powdered sugar, softened butter, cream, and mint extract. Begin mixing on low speed until the ingredients come together, then raise to medium-high and whip for about 3 minutes, until the frosting turns airy and fluffy.

If you’d like a festive touch, add a drop or two of green food coloring and mix until the color is even. Spread the buttercream evenly over the cooled brownies. Place the pan in the freezer for 20 minutes so the frosting firms up before adding the glaze.

In a small pan on low heat, gently combine the chocolate and butter, stirring until the mixture is completely melted and velvety. Remove the brownies from the freezer and carefully spread the warm glaze over the chilled buttercream layer. Smooth it out quickly with a spatula before it begins to set.

Let the glaze cool until it firms up. You can speed this along by placing the pan in the refrigerator for 5 to 10 minutes. Once the top is set, cut the brownies into squares using a sharp knife dipped in hot water for neat, clean slices.