Step 1: Preheat the Oven. Begin by preheating your oven to 350°F (175°C).
Step 2: Prepare the Loaf Pan. Line your 9x5-inch loaf pan with parchment paper, leaving a bit of an overhang on the sides.
Step 3: Mix the Brownie Batter. In a mixing bowl, combine the melted butter and granulated sugar. Whisk together until well combined. Add the egg and vanilla extract, mixing until smooth.
Step 4: Add Dry Ingredients. Sift in the cocoa powder, flour, baking powder, and salt. Gently fold the dry ingredients into the wet mixture until just combined.
Step 5: Prepare the Peanut Butter Swirl. In a separate bowl, combine the melted butter, powdered sugar, smooth peanut butter, vanilla extract, and pinch of salt. Mix until smooth and creamy.
Step 6: Layer the Batter. Pour half of the brownie batter into the prepared loaf pan. Dollop spoonfuls of the peanut butter mixture over the batter. Use a knife or toothpick to swirl the peanut butter into the brownie batter gently.
Step 7: Finish with Brownie Batter. Pour the remaining brownie batter over the peanut butter layer and add a few more dollops of the peanut butter mixture on top. Swirl again for a marbled effect.
Step 8: Bake. Bake in the preheated oven for about 20-25 minutes, or until a toothpick inserted into the center comes out with a few moist crumbs.
Step 9: Cool and Serve. Let the brownies cool in the pan for 10 minutes. Lift them out using the parchment paper overhang and transfer them to a wire rack to cool completely. Slice and enjoy!