Preheat the oven to 350°F (180°C) and position an oven rack in the middle of the oven. Line a 9 x 9-inch baking pan with parchment paper allowing the ends to extend over two of the sides to create a sling.
Cut the ½ cup unsalted butter into 1-inch pieces. Place the butter in a large microwave-safe bowl. Place the bowl in the microwave and cover. Heat the butter at 50 percent power until melted. This should take 30 to 60 seconds.
Pour the melted butter and 1 cup fresh mint leaves into a blender and process until smooth. About 1 minute.
In the same large microwave-safe bowl, combine the mint butter and 1 bag semi-sweet chocolate chips. Melt in 30-second increments at 50% power, whisking in between each increment until completely smooth.
Whisk in the ¾ cup white granulated sugar and ¼ cup brown sugar until completely combined. Whisk in the 3 large eggs, 1 egg yolk, and 1 tablespoon vanilla extract.
Add the ½ cup all-purpose flour, ¼ cup dutch process cocoa powder, and ½ teaspoon kosher salt. Fold together with a rubber spatula or wooden spoon until just combined.
Fold in the 1 cup Andes Mint pieces.
Pour the batter into the prepared baking dish. Bake for 30-40 minutes or until a toothpick inserted in the center comes out with very moist crumbs clinging to it. Remember the brownies will continue to bake as they cool.
Cool completely before using the parchment paper to lift the brownies from the pan.
Notes
Use good quality chocolate. It will make a big difference in the flavor of these brownies.For accuracy, use a kitchen scale and measure ingredients in grams. Weight measurements are more accurate than volume and will give you the most consistent results. This is my favorite kitchen scale because the display pulls out so that is easy to view when measuring in large bowls.Do not overbeat the batter once the flour and cocoa powder has been added. Over mixing traps more air creating a lighter less fudgy brownie.Line the baking pan with parchment paper, leaving an overhang on two opposite sides. This makes lifting the brownies out of the pan to cut them super easy. I like to spray the pan with non-stick spray first, which helps to hold the parchment paper in place.Err on the side of underbaked brownies rather than overbaked brownies. Brownies will continue to bake even after they have been pulled out of the oven. After they have cooled for about 15 minutes use the ends of the parchment paper to lift them out of the pan and place them on a cooling rack to continue cooling down.Be patient! Don’t start slicing into the brownie the second it comes out of the oven. It won’t be set just yet, and you will end up with a chocolatey mess.Double the recipe and bake in a 13 x 9-inch pan. The baking time may need to be adjusted. Start by baking the brownies for 35 minutes before checking for doneness.Storage: Brownies should be stored at room temperature and will stay fresh for at least 3-4 days. If you must store them for longer place them in the refrigerator for up to two weeks. The brownie will have a better flavor if you allow it to return to room temperature before eating it.