HEAT oven to 350°F.
CHOP 2 oz. chocolate. Beat cake mix, dry pudding mix, eggs, sour cream, oil and water in large bowl with mixer until blended. Stir in chopped chocolate. Pour into 2 (9-inch) round pans sprayed with cooking spray.
BAKE 30 to 35 min. or until toothpick inserted in centers comes out clean. Cool in pans 10 min. Loosen cakes from sides of pans. Invert onto wire racks; gently remove pans. Cool cakes completely.
MICROWAVE COOL WHIP and remaining chocolate in microwaveable bowl on HIGH 1-1/2 min. or until chocolate is completely melted and mixture is well blended, stirring after 1 min. Let stand 15 min. to thicken.
STACK cake layers on plate, filling and frosting with COOL WHIP mixture. Sprinkle with nuts.