Step 1: Make the Peanut Butter Dough:

In a medium-sized mixing bowl, combine the softened peanut butter, butter, and brown sugar. Use an electric mixer (handheld or stand mixer) to beat the ingredients together until the mixture is light and fluffy. This typically takes about 2-3 minutes on medium speed. Be sure to scrape down the sides of the bowl occasionally to ensure all ingredients are evenly incorporated. Add the egg and vanilla extract to the peanut butter mixture. Continue to beat until well combined, about 1 minute.

In a separate, smaller bowl, whisk together the all-purpose flour, baking soda, and salt. Whisking ensures that the baking soda and salt are evenly distributed throughout the flour, which is essential for proper leavening and flavor balance in the cookies. Gradually add the dry ingredients to the wet ingredients, mixing on low speed until just combined. Be careful not to overmix, as this can result in tough cookies. Stop mixing as soon as the flour is incorporated and the dough is uniform.

Step 2: Make the Brownie Dough:

In a separate medium-sized mixing bowl, whisk together the melted butter and granulated sugar until well combined. Ensure that the sugar is fully incorporated into the melted butter, creating a smooth and consistent mixture. Add the cocoa powder to the butter and sugar mixture and beat until smooth and well combined. This step is crucial for developing the rich, chocolatey flavor of the brownie dough. Scrape down the sides of the bowl as needed.

Beat in the egg and vanilla extract until fully incorporated. Fold in the all-purpose flour, baking powder, and salt. Mix until no dry streaks remain. Do not overmix. Then, gently fold in the chocolate chips until they are evenly distributed throughout the brownie dough.

Step 3: Chill the Doughs:

Cover both the peanut butter dough and the brownie dough with plastic wrap, pressing the wrap directly onto the surface of each dough to prevent a skin from forming. Place both bowls in the refrigerator and chill for at least 30 minutes. Chilling the dough is essential for preventing the cookies from spreading too much during baking. It also helps to develop the flavors and make the dough easier to handle.

Step 4: Shape and Swirl the Cookies:

Preheat your oven to 350°F (175°C). Line a baking sheet with parchment paper. This prevents the cookies from sticking and makes for easy cleanup.

Scoop 1-2 tablespoon portions of each dough. Roll each portion of dough into a ball. Place the peanut butter dough ball and the brownie dough ball side by side on the prepared baking sheet, leaving about 2 inches of space between each pair of dough balls. Repeat this process until all of the dough is used.

Use a toothpick or a thin skewer to gently swirl the doughs together in a marble pattern. Be careful not to overmix the doughs, as this can muddy the colors and lose the distinctive swirl effect. Aim for a visually appealing, marbled effect with distinct swirls of peanut butter and brownie dough.

Step 5: Bake and Cool:

Bake in the preheated oven for 9-11 minutes, or until the tops are lightly crackly and the edges are just set. The centers will still be quite soft, which is what gives these cookies their chewy texture.

Allow the cookies to cool on the baking sheet for 5-10 minutes before transferring them to a wire rack to cool completely. This allows the cookies to set up slightly and prevents them from breaking when you move them.