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. Set aside.
In a large mixing bowl, cream together the softened butter, granulated sugar, and brown sugar until light and fluffy.
Beat in the egg and vanilla extract until well combined.
Gradually add the dry ingredients to the wet mixture, mixing until just combined.
Fold in the chocolate chips until evenly distributed.
In a small microwave-safe bowl, warm the peanut butter for about 20 seconds until it’s easier to swirl.
Using a tablespoon, scoop out the brownie cookie dough onto the prepared baking sheet, leaving space between each scoop.
Drop small spoonfuls of peanut butter on top of each cookie dough scoop.
Using a toothpick or a knife, gently swirl the peanut butter into the brownie dough to create a marbled effect.
Bake in the preheated oven for 10-12 minutes, or until the edges are set.
Allow the cookies to cool on the baking sheet for 5 minutes before transferring them to a wire rack to cool completely.