BEAT first 5 ingredients with mixer until well blended.

CUT each cake horizontally into 2 layers. Stack on plate, spreading 3/4 cup frosting between each layer. Frost top and side with remaining frosting.

MICROWAVE chocolate and Cool Whip in microwaveable bowl on HIGH 2 min. or until chocolate is completely melted and mixture is well blended, stirring after each minute. Cool 15 min.; spread over top of cake, allowing excess to drizzle down side. Keep refrigerated.