Step 1

For an accurate measurement of boiling water, bring a full kettle to a boil and then measure out the desired amount.

Step 2

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. Push foil into corners and up sides of pan, smoothing foil flush to pan. Spray foil with vegetable oil spray.

Step 3

Whisk boiling water, unsweetened chocolate, and cocoa in large bowl until chocolate is melted. Whisk in sugar, oil, eggs and yolks, melted butter, and vanilla until combined. Gently whisk in flour and salt until just incorporated. Stir in bittersweet chocolate.

Step 4

Transfer batter to prepared pan. Bake until toothpick inserted in center 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 1½ hours.

Step 5

Using foil overhang, lift brownies out of pan. Return brownies to wire rack and let cool completely, about 1 hour. Cut into 2-inch squares and serve.