Preheat the oven to 350℉. Line a 9x9-inch pan with parchment paper, leaving the edges to hang over the sides of the pan or grease with non-stick cooking spray.

Whisk together the melted butter and sugar. Add the eggs and vanilla extract and whisk for 1 minute until smooth and frothy. Finally whisk in the flour, cocoa, baking powder, and salt just until no streaks of flour remain.

Stir together the peanut butter and powdered sugar until combined. Pour most of the brownie batter into the prepared pan, reserving about ½ cup of the batter. Spread the peanut butter/powdered sugar mixture on top of the batter in the pan. Drop spoonfuls of the reserved ½ cup of batter over the peanut butter and then run a butter knife through the batter to create swirls. Do not overmix while doing this!

Bake in the preheated oven for 25-25 minutes depending on how well done you like your brownies. Let cool for at least 10-15 minutes before serving. Enjoy!