Place an oven rack on the lowest position and preheat to 350ºF. Line a 9x13 baking pan with foil and spray with nonstick spray. Set aside.
In a large bowl, whisk together cocoa and boiling water until smooth. Add unsweetened chocolate and whisk until melted. Whisk in oil and melted butter (the mixture may look curdled). Add eggs, egg yolks, and vanilla and whisk until smooth and homogenous. Whisk in sugar until fully incorporated. Add flour and salt and fold into batter with a rubber spatula. Add bittersweet and semisweet chips and walnuts and fold gently until incorporated.
Pour batter into prepared pan and smooth the surface. Bake 30-40 minutes or until a toothpick inserted in the center comes out with moist crumbs attached. Transfer the pan to a wire rack and let cool for 1 ½ hours.
Use the foil to remove the brownies from the pan and return the brownies to the wire rack to cool completely, about 1 hour. Cut and serve. Brownies will last in an airtight container at room temperature for up to 3 days.