Preheat the oven to 350℉. Spray the bottom of a 9x13-inch baking pan with nonstick baking spray.
non-stick baking spray
Prepare the brownie mix according to the package directions using the water, oil, and eggs on the package. Spread the brownie mixture evenly in the prepared pan.
1 box fudge brownie mix, 18.3 ounce size, I used Duncan Hines, eggs, oil, and water called for on the brownie box
In a microwave safe bowl, microwave the peanut butter on high for 30 seconds. Add the powdered sugar and stir until smooth.
1 cup creamy peanut butter, ¼ cup powdered sugar, sifted
Drop the peanut butter mixture, by teaspoonfuls, onto the brownie mixture. Gently swirl the two mixtures together with a knife, making sure you have swirls throughout the pan.
Bake 29 to 33 minutes or until the center is set. Cool completely on a wire rack before cutting.