Preheat oven to 350°F and adjust the rack to the lowest position. Prepare a 13x9-inch baking dish by creating a foil sling—using two sheets of aluminum foil, one 13 inches wide and the other 9 inches wide. Lay sheets perpendicular to each other with excess foil hanging over the pan edges. Press foil into the pan, smoothing it against the bottom and sides. Spray with vegetable oil.

In a large bowl, whisk boiling water, unsweetened chocolate, and cocoa until the chocolate melts. Add sugar, oil, eggs, yolks, melted butter, and vanilla, whisking until smooth. Gently whisk in flour and salt just until combined. Stir in bittersweet chocolate chunks.

Pour batter into the prepared baking dish and bake for 30 to 35 minutes, rotating the pan halfway through. The brownies are done when a toothpick inserted into the center comes out with just a few moist crumbs. Transfer to a wire rack and let cool for 1½ hours.

Using the foil overhang, lift the brownies from the pan. Place on a wire rack and cool completely for another hour. Cut into 2-inch squares and serve.