Preheat oven to 350ºF (180ºC). Line a 9x13-inch baking pan with parchment paper, allowing excess to overhang pan edges. Set aside. In a large bowl, whisk together the cocoa powder, espresso powder (if using), and boiling water until smooth. Add chocolate chips and whisk until chocolate is melted. Whisk in melted butter and oil. (Mixture may look curdled.) Add eggs, egg yolks, and vanilla and continue to whisk until smooth. Whisk in sugar until fully incorporated.  Add flour and salt and mix with rubber spatula until combined.

Pour the batter into the prepared pan and smooth with an offset spatula. Bake until a toothpick inserted in the center comes out with just a few moist crumbs attached, 22-25 minutes. Transfer pan to wire rack and cool in the pan while you prepare the filling.

In the bowl of a stand mixer fitted with the paddle attachment, beat all ingredients at low speed until mixture starts to come together; gradually increase mixer speed to medium, and beat until smooth, 1 to 2 minutes.

Once brownies have cooled, spread the peppermint filling over the brownies in an even layer. Place brownies in the refrigerator to chill for 15 minutes.

Place the chocolate, cream, butter and salt in a microwave-safe bowl. Heat on high in 15-second increments until fully melted, stirring between. Let the topping cool for 5 minutes.

Remove the chilled brownies from the fridge and pour the ganache over top then spread with an offset spatula to form an even layer. Sprinkle the peppermint bark pieces evenly over the surface. Return to the refrigerator to chill until the chocolate is fully set, at least 45 minutes.

Use the parchment paper to lift the brownies out of the pan and use a sharp knife to cut them into 15 even pieces. For cleanest slices, dip the knife into warm water and dry with a clean towel in between each cut.