Preheat oven to 350 degrees F. Line a 9x13 metal rectangular baking pan with foil, extending the sides of the foil over the edges of the pan. Spray the foil liberally with cooking spray and set aside.

For the brownies: In a medium saucepan, melt the butter and chopped chocolate together over medium-low heat, stirring constantly for about 5 or so minutes. Remove from the heat and let stand about 10 minutes to cool slightly.

Whisk in the granulated sugar until fully combined, followed by the eggs, one at a time, mixing well after each addition, and then the vanilla extract. Lastly, add in the salt, all-purpose flour and cocoa powder and whisk until a smooth and shiny batter comes together. Pour the batter evenly into the prepared pan.

Bake the brownies for 30-35 minutes or until a toothpick inserted near the center of the brownies comes out clean or with moist, not wet, crumbs. Place the pan on a wire rack to cool completely.

For the frosting: In the bowl of a stand mixer, cream together the butter, heavy cream, and extracts until fluffy and light, about 1 minute. Add in the confectioners' sugar and mix until frosting is fluffy. Add in a drop of liquid green food coloring, if using, and mix until uniform in color.

Frost the cooled brownies evenly with the mint frosting, then cover the pan and refrigerate for at least an hour for the frosting layer to set and meld with the brownies. You can also leave the frosted brownies covered in the fridge up to 6 hours.

For the ganache: In a medium saucepan, melt the chocolate chips and cubed butter over medium-low heat, stirring constantly until melted and smooth, about 5 minutes. Remove from heat and immediately pour over the mint frosting layer evenly. Cover and immediately refrigerate for at least two hours, up to overnight, before cutting into squares.

For clean cuts: wet a large, sharp knife with hot water. Wipe the water off and make quick cuts, one cut at a time, wiping the knife off between cuts for a neat, clean cut.