Preheat your oven to 350°F (175°C). Grease an 8x8-inch baking dish or line it with parchment paper for easy removal.
In a large bowl, mix together the cocoa powder, granulated sugar, brown sugar, flour, baking powder, and salt until well combined.
Add the melted butter, eggs, and vanilla extract to the dry ingredients, and stir until just combined. Do not overmix.
Gently fold in the chocolate chips if using. Pour half of the brownie batter into the prepared baking dish and spread it evenly.
In a small microwave-safe bowl, heat the peanut butter for about 20-30 seconds until it is easy to stir and pour.
Drop spoonfuls of the warmed peanut butter over the brownie batter in the dish. Then, pour the remaining brownie batter on top.
Use a knife or skewer to swirl the peanut butter into the brownie batter, creating a marbled effect.
Bake for 25-30 minutes, or until a toothpick inserted in the center comes out with a few moist crumbs (do not overbake).
Once baked, let the brownies cool in the pan for about 10 minutes before transferring them to a wire rack to cool completely.
Notes
Cut the brownies into squares and dust with powdered sugar for a beautiful finish. Serve with a dollop of whipped cream or a scoop of vanilla ice cream for an indulgent treat!