Make the brownies according to your box directions and spread the batter evenly in your baking dish.

Microwave the peanut butter in a microwave-safe dish for 30 seconds. Add in the powdered sugar and stir until smooth.

Drop spoons of the peanut butter mixture around on the top of the brownie batter and swirl gently with a knife.

Bake according to the brownie box directions. Allow to cool before cutting.