Chop 2 oz. chocolate. Beat cake mix, dry pudding mix, eggs, sour cream, oil & water in large bowl with mixer until blended. Stir in chopped chocolate. Pour into 2 (9 in.) round pans sprayed with cooking spray.
Bake 30 to 35 minutes or until toothpick comes out clean. Cool in pans 10 minutes. Loosen cakes from sides of pans. Invert onto wire racks: gently remove pans. Cool cakes completely.
Microwave cool whip & remaining chocolate on high 1 1/2 minutes or until chocolate is completely melted & mixture is well blended, stirring after 1 minute. Let stand 15 minutes to thicken.
Stack cake layers on plate, filling & frosting with cool whip mixture. Sprinkle with nuts.