Preheat oven to 325F (or the temperature listed on your brownie mix box).
Follow the directions on the box, mixing the brownie mix with the required ingredients (typically eggs, oil, and water) until smooth.
Place parchment paper in an 8×8-inch baking pan and pour the brownie batter evenly into the pan.
Drizzle or dollop the peanut butter on top of the brownie batter. Use a toothpick or butter knife to gently swirl it through the batter for a marbled effect.
Bake the brownies for the recommended time, usually around 40 minutes, but start checking at the 30-minute mark. When a toothpick inserted about 2 inches from the edge comes out with moist crumbs (not wet batter!), they’re done.
Let the brownies cool in the pan for about 10 minutes, then transfer to a wire rack to cool completely. Once cooled, cut into squares.
Enjoy!