Preheat your oven to 350°F (175°C). Grease the baking dish with butter to prevent sticking.
In a double boiler or a microwave-safe bowl, melt the 6 oz of bittersweet chocolate chips and 1/2 cup of diced butter together until smooth. Stir occasionally to avoid burning. Remove from heat and let cool slightly.
In a large mixing bowl, combine the melted chocolate mixture with 1 1/4 cups of granulated sugar and 1/2 tsp of salt. Whisk until well combined. Add in the 3 large eggs one at a time, mixing well after each addition. Stir in 1/2 tsp vanilla extract.
Sift in 2 tbsp of cocoa powder and 2/3 cup of all-purpose flour into the chocolate mixture. Gently fold the dry ingredients into the wet ingredients until just combined. Avoid overmixing to keep the brownies fudgy.
Pour the brownie batter into the prepared baking dish, spreading it evenly. Bake in the preheated oven for about 25-30 minutes or until a toothpick inserted in the center comes out with a few moist crumbs. Allow the brownies to cool in the dish on a wire rack.
In another mixing bowl, combine 1/2 cup of softened butter, 3 oz of softened cream cheese, and 1 1/2 tsp of mint extract. Mix until creamy and well blended. Gradually add in 2 1/2 cups of powdered sugar, mixing until smooth. If desired, add 5 drops of green food coloring to achieve your preferred shade of mint green.
Once the brownie base has cooled completely, spread the mint layer evenly over the brownies. Use a spatula to create a smooth top.
In a double boiler, melt the 8 oz of semi-sweet chocolate chips with 5 tbsp of diced butter until smooth. Stir until fully combined. Allow to cool slightly.
Pour the chocolate ganache over the mint layer, spreading it evenly. Refrigerate the brownies for at least 1 hour to allow the layers to set properly.
Once set, remove the brownies from the baking dish and cut them into squares. Serve chilled or at room temperature and enjoy!