Chewy and boldly flavored, these brownies walk a thin line between sweet and salty, which is part of what makes them so addictive-eating them is like trying to have just one potato chip. This recipe needs a moderately sweet chocolate for the best texture, as the chocolates with higher percentages of cacao make the batter too dry and bitter. Save those for a recipe in which their intense flavor can shine.

Preheat the oven to 350°F and position an oven rack in the center. Line a 9 by13-inch baking pan with parchment paper, or foil, across the bottom and up two long sides, then lightly coat with melted butter, oil, or high-heat canola-oil spray.

Bring 2 inches of water to a boil in the bottom of the double boiler. Place the chopped chocolate in the top of the double boiler (off the heat). Turn off the heat, then set the chocolate over the steaming water. While you are preparing the rest of the recipe, occasionally stir the chocolate with the spatula until it is smooth and melted. Let it sit over the warm water until needed.

In the bowl of the stand mixer, cream the butter and brown sugar on medium-high speed for 5 minutes, or until much lighter in color (the mixture will look clumpy and sandy even when fully creamed). You can also use a hand mixer and a medium bowl, although you may need to beat the mixture a little longer to achieve the same results. Scrape down the sides of the bowl with the cleaned spatula. Add the peanut butter and beat well on medium-high for 30 seconds. Scrape down the bowl again. Crack the eggs into the small bowl and beat with a fork to blend. With the mixer running on medium, add the eggs, about a tablespoon at a time, incorporating each addition fully before adding the next. Beat in the vanilla. Scrape down the bowl.

In the medium bowl, whisk together the flour, baking powder, and salt. Add to the peanut butter mixture all at once, then blend on the lowest speed just until you no longer see any streaks of flour and the batter is smooth. Scrape the sides and bottom of the bowl to make sure that any patches of flour or butter are blended into the batter.

Stir half of the batter into the melted chocolate (it may look slightly grainy, but this is fine). Stir the chopped peanuts into the remaining half of the batter.

Use a small offset spatula to spread a little more than half of the plain peanut batter in the bottom of the prepared pan. This layer will be very thin, and it will seem like there is not enough batter, but it will be fine. Top with all of the chocolate batter, spreading it into an even layer (this is easiest to do by dropping big dollops of batter around the pan, then merging them with the spatula). Drop the remaining peanut batter by the level tablespoon in 3 evenly spaced rows of 5 dollops each. If you have any batter left, drop it in wherever you like. Drag a toothpick or the tip of a paring knife through each dollop a couple of times, swirling it into the chocolate batter around it. The batter will look very rough and ragged-don't worry, it will smooth out as it bakes, and the slightly rugged look that remains is very appealing.

Bake the brownies for 30 to 40 minutes, until the top is golden brown and a toothpick inserted into the center comes out with a few moist crumbs clinging to it. Transfer to a rack to cool for 15 to 20 minutes. To remove the brownies from the pan, run a thin knife or flexible spatula around the two short edges to loosen them from the pan. Grasp the parchment paper along the two long edges and pull gently upward. Set the brownies on a cutting surface and cut as desired. Serve warm.

Storing: The brownies will keep, wrapped tightly in plastic, at room temperature for 2 to 3 days or in the refrigerator for 4 or 5 days. These brownies are at their best when warm. Reheat them for 15 to 20 seconds in the microwave to restore their chewiness and bring out their flavors.