beat pudding mix and milk in a 4-1/2 qt.or larger slow cooker sprayed with cooking spray with whisk 2 min.
Prepare cake batter as directed on package:carefully pour over pudding. (Do Not Stir) Cover with lid.
cook on Low 2-1/2 to 3 hours(or on High 1-1/2 to 2 hours)or until toothpick inserted in center of cake comes out clean. Top with chopped chocolate. Turn off slow cooker. Let stand, covered, 30 min. to allow pudding to thicken. Serve with Cool Whip