Step 1: Prepare the Brownie Dough

1. In a mixing bowl, whisk together the melted butter, granulated sugar, and brown sugar until smooth.
2. Add the egg and vanilla extract, whisking until fully combined.
3. In a separate bowl, sift together the flour, cocoa powder, baking powder, and salt. Gradually fold the dry ingredients into the wet mixture until a soft dough forms. Cover and refrigerate while preparing the peanut butter dough.

Step 2: Prepare the Peanut Butter Dough

1. In another mixing bowl, beat the peanut butter, softened butter, granulated sugar, and brown sugar until creamy.
2. Add the egg and vanilla extract, mixing until smooth.
3. Combine the flour, baking soda, and salt in a separate bowl. Slowly mix the dry ingredients into the peanut butter mixture until well incorporated.

Step 3: Create the Swirl

1. Scoop 1 tablespoon of brownie dough and 1 tablespoon of peanut butter dough. Press the two pieces together gently and roll them into a ball.
2. Swirl the two doughs together by lightly rolling the ball between your hands, ensuring the colors remain distinct but blend slightly.

Step 4: Bake the Cookies

1. Preheat your oven to 350°F (175°C) and line a baking sheet with parchment paper.
2. Place the dough balls on the baking sheet, spacing them about 2 inches apart.
3. Bake for 10-12 minutes, or until the edges are set but the centers look slightly underbaked. This ensures a chewy texture.
4. Remove from the oven and let the cookies cool on the baking sheet for 5 minutes before transferring to a wire rack.