In a large bowl, combine the cake mix, water, eggs and oil; beat on low speed for 30 seconds. Beat on medium for 2 minutes. Transfer to a greased 4-qt. slow cooker.
In another bowl, whisk milk and pudding mix for 2 minutes. Let stand until soft-set, about 2 minutes. Spoon over cake batter; sprinkle with chocolate chips. Cover and cook on high for 3-4 hours or until a toothpick inserted in cake portion comes out with moist crumbs. Serve warm.