Preheat oven to 350°F and lightly grease an 8x8 baking dish with butter, oil, or cooking spray.
Put cake mix in a large bowl. Add vegetable oil and eggs and egg yolk and using a rubber spatula or wooden spoon, stir until just combined, taking care not to over-mix.
Fold in chocolate chips, if using.
Spread batter into prepared pan and bake for 20-23 minutes, until the center of the brownies in the pan is no longer jiggly and is just set to the touch. Be careful not to over-cook brownies.
Let brownies completely cool slightly before cutting.