Spray the inside of a 4-quart slow cooker with vegetable cooking spray.
Beat the cake mix, pudding mix, sour cream, eggs, vegetable juice and oil in a large bowl with an electric mixer on medium speed for 2 minutes. Stir in the chocolate pieces. Pour the batter into the cooker.
Cover and cook on LOW for 6 to 7 hours or until a knife inserted in the center comes out with moist crumbs. Serve with the raspberry topping and whipped cream.