Prepare and bake cake as directed on package for 13x9 cake pan. Cool in pan for 15 minutes. Pierce cake with large fork at ½” intervals.
In a small saucepan, combine milk, sugar and ½ cup of the coconut. Bring to a boil. Reduce heat to low; simmer one minute. Spoon over warm cake, spreading coconut evenly over top. Let cool completely.
Stir ½ cup coconut into the Cool Whip. Spread over cake. Sprinkle cake with remaining coconut. Refrigerate for 6 hours, or overnight for best results. Store any uneaten cake in refrigerator.