PREPARE cake batter and bake as directed on package for 13x9-inch pan. Cool in pan 15 min. Pierce cake with large fork at 1/2-inch intervals.
MIX milk, sugar and 1/2 cup coconut in medium saucepan. Bring to boil. Simmer on low heat 1 min. Spoon over warm cake, spreading to cover top of cake. Cool completely.
ADD 1/2 cup coconut and coconut extract to COOL WHIP; stir until blended. Spread over cake. Sprinkle with remaining coconut.
REFRIGERATE 6 hours.