⅓cupcreamy “no-stir” peanut butterheated until warm & stirred until smooth
Adjust oven rack to center position and preheat oven to 350°F. Grease an 8-inch square baking pan (or spray with nonstick cooking spray) and then line with parchment paper, making the pieces long enough to hang over the top edges of the pan.
Finely chop the chocolate, place in a microwave-safe bowl, and heat in the microwave for 45 seconds; stir. Heat for 30 more seconds and stir well, scraping the sides of the bowl. Then heat in additional 15-second increments, stirring well after each one, until chocolate is completely melted and smooth. Set aside to cool. (Alternatively, you may melt the chocolate in a double boiler on the stove.)
In a large bowl, use an electric mixer to cream the butter. Add the sugar and blend. Increase the speed to medium-high and beat for 3 to 5 minutes or until light and fluffy, scraping the bowl after the first minute. Add the eggs, one at a time, mixing well after each addition, scraping the bowl as necessary, and blending until smooth. Add the vanilla, espresso powder (if using), salt, and cocoa powder; blend for 1 minute. Scrape the bowl, add the cooled, melted chocolate, and beat on medium-high for 1 minute or until smooth. Fold in the peanut butter chips.
Hold back one big spoonful of batter and spread the remaining batter into the prepared pan, smoothing the top. Drizzle the warm peanut butter over the top of the batter in the pan, dollop small bits of the reserved brownie batter on top of the peanut butter, and swirl with a toothpick to achieve your desired design.
Rap the pan against the counter to remove any trapped air bubbles. Bake for 20 to 25 minutes or until the center is set, the brownies are puffed, and a toothpick comes out just clean, taking care not to overbake. Place the pan on a wire rack and allow to cool completely.
Once the brownies are cool, loosen the non-parchment paper edges with a plastic knife or a thin, hard, plastic spatula. Then use the parchment paper like a sling to carefully lift the slab from the pan and onto a cutting board. Use a large knife to slice into 16 brownies. Store in a sealed container at room temperature.
Chocolate chips may be used in place of chopped chocolate baking bars.