Adjust oven rack to lowest position and heat oven to 350 degrees. Make foil sling by folding 2 long sheets of aluminum foil so that they are as wide as 13 by 9‑inch baking pan (one 13‑inch sheet and one 9‑inch sheet). Lay sheets of foil in pan perpendicular to one another, with extra foil hanging over edges of pan. Push foil into corners and up sides of pan, smoothing foil flush to pan. Grease foil and set aside.

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 oil and melted butter. (Mixture may look curdled.) Add eggs, egg yolks, and vanilla and continue to whisk until smooth and homogeneous. Whisk  in sugar until fully incorporated. Whisk together flour and salt in small bowl and then mix into batter with rubber spatula until combined. Fold in bittersweet chocolate pieces.

Transfer batter to prepared pan; spread batter into corners of pan and smooth surface. Bake until toothpick inserted in center of brownies comes out with few moist crumbs attached, 30 to 35 minutes, rotating pan halfway through baking. Transfer pan to wire rack and let cool for 11/2 hours.

Remove brownies from pan using foil. Return brownies to wire rack and let cool completely, about 1 hour. Cut brownies into 2‑inch squares and serve. (Brownies can be stored in airtight container at room temperature for up to 3 days.)