I made a couple small changes because I was missing a couple of the ingredients. I used olive oil instead of vegetable oil and I left out the bittersweet chocolate and used semi-sweet chocolate chips instead.
Use 9×13 baking pan and line with parchment paper or spray lightly with oil. Preheat oven to 350 degrees.
Whisk cocoa powder and boiling water together until smooth. Add unsweetened chocolate and whisk until chocolate is melted. Whisk in melted butter and oil. 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 chocolate chips.
Pour into pan and bake for about 25 to 30 mins. Wait until they are totally cool before you cut.
Recipe by Simply Yum at https://www.simplyyum.com/2013/02/11/theperfectbrownie/