This is so easy! Been making this for many years.
Prepare and bake cake according to pkg instructions for a 13x9 inch pan.
Cool completely.
Mix pineapple and sugar in saucepan.
Bring to a boil.
Mix cornstarch and water; stir into pineapple.
Cook and stir for 1 minute; remove from heat.
Cool completely.
Cover cake top with pineapple mixture.
Top with whipped topping; sprinkle with nuts.
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