Whisk cocoa, espresso powder, if using, and 1/2 cup plus 2 tablespoons boiling water together in large bowl until smooth. Add unsweetened chocolate and whisk until smooth. Whisk in oil and melted butter. (Mixture may look curdled.) Add eggs and yolks and vanilla and whisk until smooth. Whisk in sugar until fully incorporated. Whisk together flour and salt in a small bowl and then mix into batter with rubber spatula until combined. Fold in bittersweet chocolate pieces.