Prepare cake mix as directed on package ,baking in 13 by 9-inch pan.Cool 15 minutes: then poke holes down through cake with a fork.Meanwhile,combine milk,sugar and 1/2 cup of the coconut in a saucepan. Bring to a boil,reduce heat and simmer 1 minute.Carefully spoon over warm cake,allowing liquid to soak down through holes.Cool completely. Fold 1/2 cup of the coconut into whipped topping and spread over cake.Sprinkle with remaining coconut. Chill overnight. Store leftover cake in refrigerator.