Preheat your oven to 350 F. Spray a 15x10-inch baking pan with non-stick cooking spray and line it with parchment paper. Set aside.
Add the cake mix, egg, milk, and oil to a large bowl and mix until just combined.
Stir in chocolate chips until combined.
Use a spatula to spread the batter into the prepared pan and smooth the top to create a flat layer.
Bake for 20-25 minutes or until a toothpick inserted in the center comes out clean.
Remove the pan from the oven, and allow the bars to cool in the pan completely before unmolding and cutting them.