HEAT oven to 350°F.

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 min. Stir in chopped chocolate. Pour into 2 greased 9-inch round pans.

BAKE 30 to 35 min. or until toothpick inserted in centers comes out clean. Cool in pans on wire racks 10 min. 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 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 frosting. Garnish with nuts. Keep refrigerated.