**Preheat the Oven:** Preheat your oven to 350°F (175°C). Line baking sheets with parchment paper.
**Make the Peanut Butter Dough:** In a mixing bowl, cream together butter, peanut butter, granulated sugar, and brown sugar until light and fluffy. Beat in the egg and vanilla extract. Gradually add flour, baking soda, and salt, mixing until combined. Set aside.
**Make the Brownie Dough:** In a microwave-safe bowl, melt butter and chocolate chips together in 30-second increments, stirring until smooth. Let cool slightly.
- In another bowl, whisk together sugar, eggs, and vanilla. Stir in the melted chocolate mixture.
- Gradually add flour, cocoa powder, baking powder, and salt, mixing until a soft dough forms.
**Combine the Doughs:** Scoop equal portions of peanut butter dough and brownie dough. Gently press them together, forming a single ball of dough with a marbled appearance. Do not overmix to maintain the swirl effect.
**Shape and Bake:** Place the dough balls on the prepared baking sheets, spacing them about 2 inches apart. Bake for 10-12 minutes, or until the edges are set and the centers look slightly underbaked.
**Cool:** Let the cookies cool on the baking sheet for 5 minutes before transferring them to a wire rack to cool completely.