Preheat your oven to 350°F (175°C). Grease a 9x9 inch baking pan or line it with parchment paper.
In a large mixing bowl, whisk together melted butter, sugar, and brown sugar until smooth and glossy.
Add eggs one at a time, beating well after each addition. Stir in vanilla extract and 1/2 cup peanut butter until fully incorporated.
In a separate bowl, sift together cocoa powder, all-purpose flour, salt, and baking powder.
Gradually fold dry ingredients into wet mixture using a rubber spatula. Fold just until no streaks remain.
Scrape the batter into the prepared pan and smooth the top with a spatula.
Spoon dollops of the remaining 1/2 cup creamy peanut butter over the batter. Using a toothpick or skewer, gently swirl the peanut butter through the batter to create a marbled effect.
Bake for 25-30 minutes, or until a toothpick inserted near the center comes out with a few moist crumbs but no wet batter.
Allow brownies to cool completely in the pan on a wire rack before slicing and serving.