Preheat your oven to 350°F (175°C). This ensures that your brownies bake evenly and come out perfectly fudgy.
Line the 9x5-inch loaf pan with parchment paper, leaving some overhang on the sides to make removing the brownies easier later on.
In a mixing bowl, combine the melted butter and granulated sugar. Whisk until combined and smooth. Next, add in the egg and vanilla extract, mixing until fully incorporated.
Sift in the cocoa powder, flour, baking powder, and salt. Gently fold the dry ingredients into the wet mixture using a rubber spatula until just combined. Be careful not to overmix.
In a separate small bowl, mix the melted butter, powdered sugar, peanut butter, vanilla extract, and a pinch of salt until smooth.
Pour half of the brownie batter into the prepared loaf pan, smoothing it out evenly. Dollop half of the peanut butter mixture over the batter and use a knife or skewer to create swirls. Pour the remaining brownie batter on top and repeat the dolloping and swirling with the remaining peanut butter mixture.
Place the loaf pan in the preheated oven and bake for 20-25 minutes, or until a toothpick inserted into the center comes out with a few moist crumbs. Be careful not to overbake.
Remove the brownies from the oven and let them cool in the pan for about 10 minutes. Use the parchment paper overhang to lift them out of the pan, then allow them to cool completely on a wire rack before cutting into squares. Enjoy!