Steps
1. Preheat your oven to 350°F (175°C) and grease a small baking dish.
2. In a microwave-safe bowl, melt the butter. Stir in the granulated sugar and brown sugar until well combined.
3. Add the peanut butter and vanilla extract, mixing until smooth. Then, incorporate the egg until fully blended.
4. In a separate bowl, whisk together the flour, cocoa powder, baking powder, and salt. Gradually fold the dry ingredients into the wet mixture until just combined.
5. Gently fold in the chocolate chips, reserving a few for the top.
6. Pour the batter into the prepared dish, smoothing it out evenly. Drop spoonfuls of remaining peanut butter on top and swirl with a knife for a marbled effect.
7. Sprinkle the reserved chocolate chips on top.
8. Bake for 20-25 minutes, or until a toothpick inserted comes out with a few moist crumbs. Let it cool before slicing.