Preheat your Oven to 350°F. Line a 9x13-inch Baking Dish with Parchment Paper, leaving an overhang on two sides to act as handles for easy removal. Lightly grease the Parchment Paper.

In a medium Saucepan over low heat, melt the 1 cup unsalted butter, 切成小块 (切成小块) and chopped 8 oz dark chocolate, 粗切 (可可含量60–70%) together, stirring constantly until smooth. Remove from heat and let cool for 5 minutes.

Whisk the 1 ½ cups sugar into the cooled 8 oz dark chocolate, 粗切 (可可含量60–70%) mixture until combined. Add the 3 eggs one at a time, whisking well after each addition. Stir in the 1 tsp vanilla extract.

Add the 1 cup all-purpose flour, ¼ cup unsweetened cocoa powder, and 1 cup unsalted butter, 切成小块 (切成小块) to the 8 oz dark chocolate, 粗切 (可可含量60–70%) mixture. Using a Spatula, fold gently until just combined — do not overmix. A few streaks of 1 cup all-purpose flour are fine; they'll incorporate fully.

Pour the batter into the prepared Baking Dish and spread it into an even layer with the Spatula. Bake for 22–25 minutes, or until a toothpick inserted in the center comes out with a few moist crumbs (not wet batter). Do not overbake.

Transfer the Baking Dish to a Wire Rack and let the brownie layer cool completely to room temperature, at least 45 minutes. The brownies must be fully cool before adding the mint layer.

Using a Hand Mixer on medium speed, beat the softened 1 cup unsalted butter, 切成小块 (切成小块) until light and fluffy, about 2 minutes. Add the sifted 1 ½ cups sugar one cup at a time, mixing on low after each addition.

Add the 3 tbsp heavy cream and 1 ½ tsp peppermint extract, then beat on medium-high for 1–2 minutes until smooth and fluffy. Add the 2 green food coloring (if using) and mix until the color is uniform. Taste and adjust 1 ½ tsp peppermint extract to your preference.

Spread the mint buttercream evenly over the cooled brownie layer using an Spatula. Place the Baking Dish in the refrigerator for 20 minutes to firm up the buttercream before adding the ganache.

Place the chopped 8 oz dark chocolate, 粗切 (可可含量60–70%) in a heatproof Mixing Bowl. Heat the 3 tbsp heavy cream in a small Saucepan over medium heat until it just begins to simmer (do not boil). Pour the hot 3 tbsp heavy cream over the 8 oz dark chocolate, 粗切 (可可含量60–70%) and let sit undisturbed for 2 minutes.

Stir the ganache gently from the center outward until completely smooth and glossy. Let it cool for 5–8 minutes until it's pourable but not hot — it should be the consistency of thick syrup.

Remove the Baking Dish from the refrigerator. Pour the ganache over the mint buttercream layer and gently tilt the Baking Dish or use an Spatula to spread it into an even layer all the way to the edges.

Return the Baking Dish to the refrigerator for at least 30 minutes, or until the ganache is fully set and firm to the touch.

Use the Parchment Paper overhang to lift the entire slab out of the Baking Dish and onto a Cutting Board. For clean cuts, run a sharp Knife under hot water, wipe dry, and slice into 16 squares. Repeat the warm-wipe between each cut.