Mix cake mix and pudding: Preheat oven to 350°F. Lightly coat a 13-x9-inch baking dish with cooking spray. Dump cake mix and instant pudding in even layer in prepared baking dish; set aside.
Warm milk and melt butter: Microwave milk and butter in a medium, heat-proof bowl on HIGH, until butter is melted, 1 to 2 minutes, stirring after 1 minute. Stir in vanilla, if using. Drizzle butter mixture evenly over dry ingredients.
Mix batter: Gently mix to break down any large clumps or dry spots.
Add chocolate chips: Sprinkle chocolate chips evenly over top of cake batter.
Bake cake: Bake in preheated oven until a wooden pick inserted in center comes out with a few moist crumbs, about 45 minutes. Let cool completely in pan on wire rack, about 1 ½ hours.
Serve: Slice and serve with whipped cream or ice cream.
