Preheat your oven to 325°F (160°C). Lightly spray an 8 or 9-inch square baking dish with cooking spray, then line it with parchment paper, allowing some overhang on the sides for easy lifting. Set aside.
In a heat-safe bowl, gently melt the ½ cup unsalted butter, unsweetened chocolate, and optional Andes baking chips together. You can do this using a double boiler or carefully in the microwave (heating in 20-second bursts and stirring well after each). Once smooth, remove from heat.
Stir the granulated sugar, vanilla extract, and ½ teaspoon mint extract into the warm chocolate mixture until well combined. Beat in the 2 large eggs one at a time, ensuring each is fully incorporated before adding the next.
Gently mix in the all-purpose flour and salt until the batter is smooth. Be careful not to overmix!
Pour the brownie batter evenly into your prepared dish. Bake for about 25 to 30 minutes, or until a toothpick inserted into the center comes out with a few moist crumbs. Place the pan on a wire rack and let the brownies cool completely. This step is crucial for perfect layers!
In a separate mixing bowl, beat the ½ cup softened butter until it's light and fluffy. Gradually add the confectioners’ sugar, ½ teaspoon mint extract, water, and green food coloring (if you're using it). Mix until the filling is wonderfully creamy and smooth.
Evenly spread the creamy mint filling over the top of the completely cooled brownie base. Carefully place the dish into the refrigerator while you prepare the ganache.
Place the 1 cup of chopped bittersweet chocolate in a medium, heat-safe bowl. In a small saucepan or microwave, heat the heavy whipping cream until it just begins to simmer – watch it closely so it doesn't boil over.
Pour the hot cream directly over the chocolate and let it sit undisturbed for 2 minutes. Then, gently whisk the mixture until the ganache is beautifully smooth and shiny. Let the ganache cool in the bowl for about 10 to 20 minutes; it should still be pourable but slightly thickened.
Carefully pour the slightly cooled ganache over the chilled mint layer, spreading it evenly to cover. If you plan to add extra Andes mint chips on top, sprinkle them now, allowing the ganache to slightly set first (about 10 minutes).
Place the finished brownies back into the refrigerator for at least 30 minutes (or longer) to allow all the layers to set firmly. Once set, use the parchment paper overhang to lift the brownies from the pan. For clean slices, use a warm, clean knife, wiping it between each cut. Cut into small, rich squares and enjoy this delightful treat!