HEAT oven to 350°F. Lightly grease two 9-inch round pans. Beat first 6 ingredients in large bowl with mixer on low speed just until moistened. Scrape bowl. Beat on medium speed 2 min. or until well blended. Stir in 2 chopped chocolate squares. Pour into prepared pans.

BAKE 30 to 35 min. or until toothpick inserted in centres comes out clean. Cool in pans 10 min. Loosen cakes from sides of pans; invert onto racks. Remove pans. Cool cakes completely.

MICROWAVE remaining chocolate squares and Cool Whip in medium microwaveable bowl on HIGH 30 sec. to 2 min. or until chocolate is almost melted; stir until completely melted. Fill and frost cake layers with Cool Whip mixture. Garnish with nuts.