Preheat your oven to 350°F (175°C). Grease and line a 9x9 inch baking pan with parchment paper.

In a mixing bowl, whisk together the melted butter and sugar until well combined.

Add the eggs and vanilla extract, and mix until smooth.

In another bowl, sift together the cocoa powder, flour, salt, and baking powder.

Gradually add the dry ingredients to the wet ingredients, mixing until just combined.

In a small bowl, mix the creamy peanut butter with the tablespoon of sugar until smooth.

Pour the brownie batter into the prepared baking pan and spread evenly.

Dollop the peanut butter mixture over the brownie batter and use a knife or spatula to swirl it gently into the batter.

Bake in the preheated oven for 25 minutes or until a toothpick inserted into the center comes out with a few moist crumbs.

Allow to cool in the pan before slicing into squares.