For the chewiest texture, it is important to let the brownies cool thoroughly before cutting. If your baking dish is glass, cool the brownies 10 minutes, then remove them promptly from the pan (otherwise, the superior heat retention of glass can lead to overbaking). Our preferred brands of bittersweet chocolate are Callebaut Intense Dark Chocolate L-60-40NV and Ghirardelli Bittersweet Chocolate Baking Bar. Our preferred brand of unsweetened chocolate is Scharffen Berger. Leftovers can be stored in an airtight container at room temperature for up to 4 days.
1/3 cup Dutch-processed cocoa
1½ teaspoons instant espresso (optional)
½ cup plus 2 tablespoons boiling water
2 ounces unsweetened chocolate, finely chopped
½ cup plus 2 tablespoons vegetable oil
4 tablespoons (½ stick) unsalted butter, melted
2 Large eggs
2 large egg yolks
2 teaspoons vanilla extract
2½ cups (17 ½ ounces) sugar
1 ¾ cup (8 ¾ ounces) all-purpose flour
¾ teaspoon salt
6 ounces bittersweet chocolate, cut into ½-inch pieces
Adjust oven rack to lowest position and heat oven to 350 degrees. Make foil sling for 13 by 9- inch baking pan by folding 2 long sheets of aluminum foil; first sheet should be 13 inches wide and second sheet should be 9 inches wide. Lay sheets of foil in pan perpendicular to each other, with extra foil hanging over edges of pan, smoothing foil flush to pan, and grease foil.
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.) Whisk in eggs, yolks, and vanilla 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.
Scrape batter into prepared pan and smooth top. Bake brownies until toothpick inserted half-way between edge and center comes out with a few moist crumbs attached, 30 to 35 minutes. Let brownies cool in pan for 1½ hours.
Using foil overhang, lift brownies from pan. Transfer to wire rack and let cool completely, about 1 hour. Cut into 2-inch squares and serve.