6 ounces bittersweet chocolate, cut into 1/2-inch pieces
For the Filling:
2 1/2 cups powdered sugar
2 tablespoons butter, softened
3 tbsp whipping cream
4 oz cream cheese, softened
1/2 teaspoon mint extract
2 drops green food color
For the Chocolate Ganache:
1/3 cup whipping cream
1 1/3 cups (8 oz) semisweet chocolate chips
1/3 cup unsalted butter
Preheat oven to 350F. Butter the bottom and sides of a 9×13 inch metal baking pan. Put a long piece of parchment paper in the bottom of the pan, letting the parchment extend up two sides of the pan and overhang slightly on both ends. Place another piece of parchment paper in the same manner, perpendicular to the first sheet. (This will make it easy to remove the bars from the pan after they have baked.) Butter the parchment.
In a large bowl, whisk cocoa, espresso powder, and boiling water together in large bowl until smooth. Add unsweetened chocolate and whisk until chocolate is melted. Whisk in melted butter and oil. Add eggs, yolks, vanilla, and peppermint and continue to whisk until smooth and homogeneous. Whisk in sugar until fully incorporated. Add flour and salt and mix with rubber spatula until combined. Fold in chocolate chunks.
Transfer the batter to prepared pan, smoothing the top. Bake 30 to 35 minutes, or until toothpick inserted into the center comes out with just a few moist crumbs attached. Transfer pan to wire rack and cool 1½ hours.
Using parchment overhang, lift brownies from pan. Return brownies to wire rack and let cool completely, about 1 hour.
To make the Filling: Combine all ingredients in the bowl of a stand mixer fitted with the paddle attachment and beat until smooth and fluffy, about 2 minutes. Spread the mint filling over the cooled brownies.
To make the Chocolate Ganache: Place all ingredients in a microwave safe bowl and microwave until the chocolate chips start to melt (about 1 minute) then stir. Microwave a little longer and stir until smooth. Let cool 5 minutes, then pour over the top of the mint filling, spreading carefully with a knife to the edges. Place the pan in the refrigerator until set, about an hour. Before cutting into bars, let stand 10 minutes at room temperature.