Preheat your oven to 350°F (175°C). Line an 8x8 inch baking pan with parchment paper, leaving some overhang on the sides to lift the brownies out later. Greasing the pan first helps the parchment stick.
In a medium saucepan over low heat, melt the butter and unsweetened chocolate, stirring constantly until smooth. Remove from heat.
Stir in the granulated sugar, vanilla extract, and mint extract.
Whisk in the eggs one at a time, mixing until just combined after each addition. Don't overbeat here!
Add the flour and salt. Stir gently with a spatula or wooden spoon until just combined. A few streaks of flour are okay – you don't want to overmix, which can make your brownies tough. If using, stir in the ¼ cup Andes chips now.
Pour the batter into the prepared pan and spread evenly.
Bake for 25-30 minutes, or until a toothpick inserted into the center comes out with moist crumbs attached (not wet batter). Remember, slightly underbaked is better than overbaked for fudgy brownies!
Let the brownies cool completely in the pan on a wire rack before adding the next layer. This is crucial! Pop them in the fridge for an hour to speed this up if you're impatient (I won't tell!).
While the brownies cool, make the filling. In a medium bowl, beat the softened butter until creamy using an electric mixer (hand or stand).
Gradually add the confectioners' sugar, mixing on low speed until incorporated, then increase speed and beat until light and fluffy.
Beat in the mint extract and water until smooth and spreadable.
Add the green gel food coloring, one drop at a time, mixing until you get the desired shade of mint green.
Once the brownies are completely cool, spread the mint filling evenly over the top using an offset spatula or the back of a spoon.
Refrigerate the brownies for at least 30 minutes to allow the mint layer to firm up.
In a heatproof bowl, place the bittersweet chocolate chips or chopped chocolate.
In a small saucepan, heat the heavy cream over medium heat just until it starts to simmer around the edges – don't let it boil rapidly.
Pour the hot cream over the chocolate. Let it sit undisturbed for 5 minutes to allow the chocolate to melt.
After 5 minutes, gently whisk the mixture starting from the center and working your way out until it's completely smooth and glossy.
Let the ganache sit at room temperature for about 5-10 minutes, stirring occasionally, until it's slightly thickened but still pourable.
Remove the chilled brownies with the mint layer from the fridge.
Pour the slightly cooled ganache evenly over the mint layer. Gently tilt the pan or use a spatula to spread it to the edges if needed, but try not to disturb the mint layer underneath.
If using, immediately sprinkle the ½ cup Andes chips over the warm ganache.
Refrigerate the brownies for at least 1-2 hours (or until the ganache is firm) before lifting them out of the pan using the parchment overhang.
Cut into squares using a sharp knife wiped clean between cuts for nice, neat edges.
Enjoy!