In a medium bowl, whisk together flour, cocoa powder, baking soda, and salt.
Cream together the softened butter with granulated sugar and brown sugar until light and fluffy. Beat in the egg and vanilla extract. Gradually add the dry mixture into this creamy base, mixing only until just combined.
Drop dollops of creamy peanut butter over your chocolate dough and swirl it in with a knife or skewer.
Cover your bowl with plastic wrap and refrigerate for at least one hour.
Preheat your oven to 350°F (175°C) and line two baking sheets with parchment paper. Scoop tablespoonfuls of dough onto the sheets and bake for about 10 to 12 minutes until set but still soft.
Let the cookies cool on the baking sheet for around five minutes before transferring them onto a wire rack.