Preheat oven to 350 degrees F. Grease an 11x7-inch baking dish with nonstick cooking spray, or line with parchment paper or foil.

In a large mixing bowl, beat together cake mix, peanut butter, eggs and melted butter with an electric mixer on medium speed. Stir in chocolate chips. Batter will be very thick.

Pour batter into the prepared baking dish and spread evenly, pressing lightly with your hands. Sprinkle on additional chocolate chips, if desired.

Bake in the preheated oven for 20-25 minutes, or until a toothpick inserted in the center comes out clean. (The center will still look underbaked).

Cool completely on a wire rack before cutting into squares and serving.

These brownie bars are especially ooey-gooey when served warm with a big scoop of vanilla ice cream and go great with a tall glass of milk!