Preheat your oven to 350°F (175°C) and grease an 8x8 inch baking dish or line it with parchment paper for easy removal.

In a medium microwave-safe bowl, melt your butter. Let it cool slightly.

In another bowl, whisk together the sugar, eggs, and vanilla until smooth. Gradually add in the cocoa powder, flour, salt, and baking powder, mixing until just combined.

Pour the melted butter into the brownie mixture and stir until everything is well integrated.

In a separate bowl, warm the peanut butter slightly in the microwave for about 20 seconds to make it easier to swirl.

Pour half of the brownie batter into the prepared dish, followed by dollops of peanut butter. Top it off with the remaining brownie batter and swirl the peanut butter lightly with a knife.

Bake in the preheated oven for 25-30 minutes or until a toothpick inserted into the center comes out slightly moist (not wet).

Allow the brownies to cool in the pan for about 10 minutes before transferring them to a wire rack to cool completely. Slice and enjoy!