1. Preheat oven to 350 and grease a 9x13 pan with nonstick cooking spray. Set aside.
2. In a mixing bowl, combine and stir cake mix, milk, oil, egg and vanilla and mix well.
3. Dough will be stiff and sticky.
4. Fold in chocolate chips.
5. Spoon batter into prepared pan and spread evenly.
6. Bake for 23-26 minutes or until browned and edges are set.
7. Cool completely before cutting into bars.