Preheat the oven to 350°F (175°C) and line two large baking sheets with parchment paper to prevent sticking and ensure even browning.

In a medium bowl, whisk together the flour, cocoa powder, baking soda, and salt.

In another bowl, beat softened butter with both sugars until light and fluffy, then add the egg and vanilla extract. Gradually mix in the dry ingredients until just combined and set aside.

For the peanut butter dough, whisk flour, baking soda, and salt in one bowl.

In a large bowl, cream butter with both sugars until fluffy, then mix in peanut butter until smooth.

Add the egg and vanilla, followed by the dry ingredients, mixing just until combined.

Take about two tablespoons of each dough and gently roll them together to form a ball, leaving visible chocolate and peanut butter swirls.

Place on the baking sheets about two inches apart. Bake for 10–12 minutes, until edges are set and centers look slightly soft.

Let cool on the baking sheet for 5 minutes, then transfer to a wire rack to finish cooling. If the dough feels too sticky to handle, chill it for 15–20 minutes before shaping.