1. Adjust oven rack to lowest position and reheat oven to 350°.
2. Line a 13x9" baking pan with foil, pushing foil into corners and up sides of pan with excess overhanging pan edges. Spray foil with nonstick cooking spray.
3. In a large mixing bowl, whisk together cocoa, espresso powder and boiling water until smooth.
4. Add unsweetened chocolate; whisk until chocolate is melted.
5. Whisk in melted butter and oil (mixture may look curdled).
6. Add eggs, yolks and vanilla; continue whisking until mixture is smooth and homogeneous.
7. Whisk in sugar until fully incorporated.
8. Add flour and salt; mix with a rubber spatula until combined.
9. Fold in bittersweet chocolate pieces.
10. Scrape batter into prepared pan; bake until a toothpick inserted halfway between edge and center comes out with just a few moist crumbs attached (30 to 35 min.).
11. Transfer pan to wire rack; let cool for 1 ½ hrs.
12. Using foil, overhand lift brownies from pan; return brownies to wire rack to cool completely (about 1 hr.).
13. Cut into 2" squares before serving.

Recipe courtesy of Cook's Illustrated, March-April 2010.