Preheat oven to 350 F. Line a 9x13 baking pan with foil, then spray. Whisk cocoa and boiling water in large bowl until smooth. Add unsweetened chocolate and whisk until chocolate is melted. Whisk in melted butter and oil. (Mixture may look curdled.) Add eggs, yolks, and vanilla, and continue to whisk until smooth. Whisk in sugar until fully incorporated. Add flour and salt, and mix with rubber spatula until combined. Fold in semisweet chocolate pieces.

Scrape batter into prepared pan and bake until toothpick inserted in center comes out with just a few moist crumbs attached, 30-35 minutes. Transfer pan to wire rack and cool 1½ hours.

Remove brownies from pan and place on wire rack to cool completely, about 1 hour. Cut into squares and serve. Brownies can be stored in an airtight container at room temperature for up to 4 days.