6 oz bittersweet chocolate, cut into ½-inch pieces
For the Peanut Butter Swirl: Mix together all the peanut butter swirl ingredients until smooth; set aside.
For the Brownies: Adjust oven rack to lowest position and heat oven to 350°F. Line a 9 by 13-inch pan with parchment paper.
Whisk cocoa, espresso powder (if using), and boiling water together in large bowl until smooth. Add unsweetened chocolate and whisk until chocolate is melted. Whisk in melted butter and oil. (Mixture may look curdled.) Add eggs, yolks, and vanilla and continue to whisk until smooth and homogeneous. Whisk in sugar until fully incorporated. Add flour and salt and mix with rubber spatula until combined. Fold in bittersweet chocolate pieces.
Scrape half the batter into prepared pan and dollop with half the peanut butter swirl mixture. Scrape the remaining batter in the pan and dollop with the remaining peanut butter swirl mixture. Using a knife, swirl the peanut butter mixture throughout brownie batter. Bake until toothpick inserted halfway between edge and center comes out with just a few moist crumbs attached, 35 to 40 minutes. Transfer pan to wire rack and cool 2 hours.
Recipe by Laura's Sweet Spot at http://laurassweetspot.com/2013/02/18/peanut-butter-swirl-brownies/