Step 1

Adjust oven rack to lowest position and heat oven to 350 degrees. Referring to directions in Making a Foil Sling (related), make sling using the following steps: Cut 18-inch length foil and fold lengthwise to 8-inch width. Fit foil into length of 13 by 9-inch baking pan, pushing it into corners and up sides of pan; allow excess to overhang pan edges. Cut 14-inch length foil and fit into width of pan in the same manner, perpendicular to the first sheet (if using extra-wide foil, fold second sheet lengthwise to 12-inch width). Spray with nonstick cooking spray.

Step 2

Whisk cocoa, espresso powder (if using), and boiling water together in large bowl until smooth. Add unsweetened chocolate and whisk until chocolate is melted. Whisk in melted butter and oil. (Mixture may look curdled.) Add eggs, yolks, and vanilla 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 bittersweet chocolate pieces.

Step 3

Scrape batter into prepared pan and bake until toothpick inserted halfway between edge and center comes out with just a few moist crumbs attached, 30 to 35 minutes. Transfer pan to wire rack and cool 1½ hours.

Step 4

Using foil overhang, lift brownies from pan. Return brownies to wire rack and let cool completely, about 1 hour. Cut into 2-inch squares and serve.