BEAT pudding mix, sour cream and milk in large bowl with whisk 2 min. Pour into slow cooker sprayed with cooking spray.
PREPARE cake batter as directed on package; slowly pour over pudding in slow cooker. (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 heat. Let slow cooker stand, covered, 30 min. or until pudding is thickened.
SERVE topped with COOL WHIP.