2 ounces unsweetened chocolate (such as Baker’s Baking Chocolate), finely chopped (or 6 T cocoa powder and subtract 2 T oil from recipe)
1/2 cup plus 2 Tablespoons vegetable oil
4 Tablespoons (1/2 stick) unsalted butter, melted
2 large eggs
2 large egg yolks
2 teaspoons vanilla extract
2.5 cups (17 ½ ounces) sugar, not brown sugar
1 3/4 cups (8 ¾ ounces) all-purpose flour
3/4 teaspoon salt
6 ounces bittersweet chocolate (70%), cut into ½-inch pieces (or 6 T cocoa powder and add 2 t butter to recipe)
Adjust oven rack to lowest position and heat oven to 350 degrees. Grease a 13×9 inch baking dish.
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.