Directions

1. Preheat your oven to 350°F. Grease a 9x5-inch loaf pan with cooking spray and set aside.
   
2. In a medium bowl, combine sugar, 4 tablespoons butter, egg, and 1/2 teaspoon vanilla. Whisk until well-combined. To the same bowl, add flour, cocoa powder, baking powder, and 1/8 teaspoon salt. Stir until just combined.
   
3. Transfer mixture to your prepared pan and use a knife to spread the batter so that it covers the bottom of the pan.
   
4. In a small bowl, whisk together peanut butter, 1 tablespoon butter, powdered sugar, 1/8 teaspoon vanilla, and dash of salt until well-mixed. Drop the peanut butter mixture by the spoonful over the brownie batter.
   
5. Use a knife to swirl in the peanut butter by drawing 3 horizontal lines down the long side and then making an "S" shape across them. Bake for 22 to 25 minutes, until a toothpick inserted into a non-peanut butter section near the middle comes out with just a few crumbs.
   
6. Place brownie pan on a cooling rack and cool for at least 15 minutes before removing the brownies. Slice and enjoy!