Do not preheat oven.
Spray a 12-cup Bundt pan with vegetable cooking spray.
In a large bowl, combine the cake mix, cocoa, sugar, sour cream and oil.
Beat with an electric mixer on low speed just long enough to blend.
Add the eggs and blend well, and then increase the speed to medium-high and beat for 4 minutes.
Fold the chocolate chips into the batter with a spatula and stir gently until they are well distributed.
Pour the batter into the prepared pan and set the oven to 325 degrees.
Bake until a cake tester or wooden toothpick inserted in the middle of the cake comes out clean (about 55 minutes.) Let the cake cool in the pan at least 1 hour before slicing.
You may drizzle chocolate icing over the cake to make it even chocolatier, but it is wonderful without it too.