Place rack on middle rack in oven. Preheat oven to 350. Make a foil liner for a 9x 13 pan by turning the baking pan pan upside down. Tear a piece of foil the length of the pan, leaving extra (about 2 inches) hanging down on both ends. Form the foil over the outside of the pan, Then flip the pan over, and place the molded foil to the inside of the pan. Spray the foil with cooking spray. Whisk cocoa and boiling water together in a large bowl until smooth. Add unsweetened chocolate and whisk until chocolate is melted. Whisk in melted butter and oil. Mixture may look curdled at this point. Add eggs, egg yolks and vanilla. Continue to whisk until smooth. Whisk in sugar until fully incorporated. Using rubber spatula, fold in flour, salt and bittersweet chocolate all at once, taking care to not over mix. Scrape batter into prepared pan and bake until toothpick inserted in center comes out with a few moist crumbs attached, about 30-40 minutes. Do not over cook. Transfer pan to wire rack and let cool completely, about 1 to 1 1/2 hours. Remove from pan using foil liner to lift brownies out of pan. Cut when cooled.