Preheat oven and prepare a 9x13 inch baking pan by greasing and flouring it.

Prepare yellow cake mix according to package instructions.

Bake cake as directed on box.

While cake is baking, heat crushed pineapple with its juice and sugar in a saucepan over medium heat.

Stir until sugar is completely dissolved.

Remove cake from oven and immediately punch holes throughout the top of the cake.

Pour the hot pineapple-sugar mixture evenly over the cake, allowing it to soak in.

Refrigerate cake until completely cool.

Prepare instant vanilla pudding according to package directions.

Spread the pudding evenly over the cooled cake.

If desired, sprinkle some frozen coconut over the pudding layer.

In a separate bowl, mix the remaining frozen coconut (if using) with Cool Whip.

Spread the coconut-Cool Whip mixture evenly over the pudding layer.

Chill cake in the refrigerator for at least 30 minutes before serving.