Preheat the oven to 350 degrees F. Spray an 8x8 inch pan with nonstick baking spray. Place a strip of parchment paper in the pan to cover the bottom and leave a little overhang on two of the sides to use as handles to pull the brownies out of the pan once cooled. Set aside.
Add the 3/4 cup butter and 4 ounces semi-sweet baking chocolate to a microwave-safe bowl. Microwave in 30-second intervals and stir after each interval until the chocolate is completely melted.
Pour the butter and chocolate into a large mixing bowl. Add in the 3/4 cup granulated sugar1/2 cup light brown sugar, 2 large eggs, and 2 teaspoons vanilla extract. Using a hand mixer, beat on medium for about 1 to 2 minutes until lighter in color and thick.
Add in the 1/2 cup unsweetened cocoa powder, 1/2 cup all-purpose flour, 1/2 teaspoon baking powder, and 1/2 teaspoon salt. Beat just until the flour is incorporated.
Add dollops of the brownie batter to the prepared pan. Gently spread the brownie batter out evenly using a spatula.
Place in the preheated oven and bake for 28 to 35 minutes until done.
Let cool completely before making the frosting.
Frosting
Add the 1/2 cup butter, 1/2 teaspoon mint extract, 1/2 teaspoon vanilla extract, 1 1/2 cups powdered sugar, 1/2 tablespoon whole milk, and 1 - 3 drops leaf green gel food color to a large bowl. Beat with a hand mixer on low just until the powdered sugar is combined.
Add in the 1/4 cup mini chocolate chips and turn the mixer up to medium/high. Beat until fluffy.
Gently spread the frosting evenly on top of the brownies.
Pull the brownies out of the pan using the parchment paper and cut.
Notes
I doubled the frosting recipe for a thicker layer.
If you want your frosting sweeter, add an additional 1/2 to 1 cup of powdered sugar and 1/2 to 1 tablespoon of milk to the frosting recipe. Taste to make the mint is still to your liking, if not, add an additional 1/4 to 1/2 teaspoon of mint extract.