CHOP 2 chocolate squares. Beat cake mix, dry pudding mix, eggs, sour cream, oil and water in large bowl with mixer on low speed just until moistened. Beat on medium speed 2 minute Stir in chopped chocolate. Pour into 2 greased 9-inch round pans.
BAKE 30 to 35 minute or until toothpick inserted in centers comes out clean. Cool in pans on wire racks 10 minute Loosen cakes from sides of pans. Invert onto racks; gently remove pans. Cool cakes completely.
PLACE COOL WHIP and remaining chocolate squares in microwaveable bowl. Microwave on HIGH 1-1/2 minute or until chocolate is completely melted and mixture is well blended, stirring after 1 minute Let stand 15 minute to thicken. Stack cake layers on plate, filling and frosting with COOL WHIP frosting. Garnish with nuts. Keep refrigerated.