Preheat your oven to 350°F (175°C) and line a baking sheet with parchment paper.
In a medium bowl, whisk together the flour, baking soda, salt, and cocoa powder.
In a large bowl, cream together the softened butter, granulated sugar, and brown sugar until light and fluffy.
Add the peanut butter, egg, and vanilla extract, mixing until well combined.
Gradually add the dry ingredients to the wet mixture, stirring until just combined.
If using, fold in the chocolate chips.
Scoop tablespoon-sized portions of dough onto the prepared baking sheet, spacing them about 2 inches apart.
Drop a small spoonful of peanut butter on top of each cookie dough ball.
Use a toothpick or knife to swirl the peanut butter into the dough, creating a marbled effect.
Bake in the preheated oven for 10-12 minutes, or until the edges are set but the centers are still soft.
Allow the cookies to cool on the baking sheet for 5 minutes before transferring them to a wire rack to cool completely.