Bake in a 9x13 pan, or a bundt pan. For rectangular pan, grease the bottom of pan only. For a bundt pan, lightly coat the inside of pan with cooking spray, and coat with flour, shaking out any excess flour. Preheat oven to 350.
In a large bowl, combine the cake mix, pudding, eggs, sour cream, warm water, and oil. Beat on low for 1 minute, then scrape the sides of the bowl with a spatula. Increase speed to medium and beat for 2-3 minutes. Gently fold in chocolate chips, distributing evenly throughout batter.
Scrape the batter into the prepared pan, smoothing the top with a spatula. For the 9x13 pan, bake for 35-40 minutes, until toothpick inserted in center comes out clean. For a bundt pan, Bake 45-50 minutes or until it just starts to pull away from the sides of the pan and springs back when lightly pressed.
(9x13) Cool completely and frost
(Bundt pan) Cool on wire rack for 20 minutes. Carefully run a sharp knife around the edge of the cake pan. Invert the cake onto the rack to cool completely. To frost, set inverted cake onto a platter or cake plate and spread the warm frosting over the cooled cake with smooth strokes. Set aside until the frosting has set, about 20 minutes.
Use a can of chocolate frosting, or make your own. This frosting recipe makes about 4 cups of frosting, so you can divide the recipe if you need less.
To make frosting: Melt butter in a medium saucepan over low heat. Add cocoa and milk, stirring constantly until the mixture comes to a boil. Immediately remove the pan from the heat. Add powdered sugar and stir unil the frosting is thick and smooth. Use immediately while frosting is warm, or it will harden. To reheat, return the pan to low heat and stir until frosting softens.