In a large bowl, cream together the peanut butter, softened butter, granulated sugar, and brown sugar until light and fluffy. Add in the egg and vanilla extract, beating until well combined. In a separate bowl, whisk the flour, baking soda, and salt, then gradually incorporate into the wet mixture. Mix until a soft dough forms. Cover and refrigerate for 30 minutes.

In a microwave-safe bowl, melt the chocolate chips and butter in 30-second intervals, stirring each time until smooth. Let cool slightly. In another bowl, whisk together sugar, egg, and vanilla. Stir this into the melted chocolate. Sift in the flour, cocoa powder, and salt, folding gently to combine. The batter should be thick and glossy.

Preheat your oven to 350°F (175°C). Line a baking sheet with parchment paper. Scoop tablespoon-sized portions of both doughs, rolling each into balls. Flatten slightly with your palm.

Press one peanut butter piece and one brownie piece together, then twist gently to marble the dough. Roll slightly into a ball if needed, then place on the baking sheet. Leave about 2 inches between cookies.

Bake for 8–10 minutes, until the edges are set but the centers remain soft. Let them rest on the baking sheet for 5 minutes before transferring to a wire rack to cool fully. The swirl pattern will look even more beautiful once baked.