Peanut Butter Swirl Brownies

1. Preheat oven to 350 degrees F. Grease a 13-inch by 9-inch metal baking pan or line the pan with parchment paper.
2. Prepare brownie batter according to box directions. Set aside.
3. Prepare peanut butter swirl: in a medium bowl, with a mixer at medium speed, beat peanut butter, sugar, butter, flour, vanilla, and egg until well blended.
4. Spread brownie batter into the pan.
5. “Plop” tablespoons of the peanut butter swirl batter evenly over brownie batter.
With a butter knife or toothpick swirl the two batters together (do not mix completely).
6. Bake brownies for 30-35 minutes. Cool in pan on a wire rack before cutting into squares.