Ingredients:

1/3 cup Dutch-processed cocoa
1 ½ tsp instant espresso (optional)
½ cup plus 2 TBS boiling water
2 ounces unsweetened chocolate, finely chopped
½ cup plus 2 TBS vegetable oil
4 TBS (1/2 stick) unsalted butter, melted
2 large eggs
2 large egg yolks
2 tsp vanilla extract
2 ½ cups (17 ½ ounces) sugar, not brown sugar
1 ¾ cups (8 ¾ ounces) all-purpose flour
¾ tsp salt
6 ounces bittersweet chocolate, cut into ½-inch pieces

Method:

Adjust oven rack to lowest position and heat oven to 350 degrees. Make foil slices for 13 by 9-inch baking pan by folding 2 long sheets of aluminum; first sheet should be 13 inches wide and the second sheet should be 9 inches wide. Lay sheets of foil in pan perpendicular to each other, with extra foil hanging over the edges of pan. Push foil into corners and up sides 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 halfway between edge and center comes out with a few moist crumbs attached, 30 to 35 minutes. Let brownies cool in pan for 1 ½ 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.