HEAT oven to 350°F.
PREPARE cake batter as directed on package. Chop 2 oz. chocolate; stir into batter. Pour into 13x9-inch pan sprayed with cooking spray.
BAKE 30 min. or until toothpick inserted in center comes out clean. Cool completely.
COMBINE coconut and condensed milk; spread over cake. Chop remaining chocolate; whisk into COOL WHIP in microwaveable bowl. Microwave on HIGH 1 min. or until chocolate is completely melted and mixture is well blended, stirring every 30 sec. Spread over cake.