Preheat oven to 350 degrees F. Spray a 13×9 inch metal cake pan with cooking spray.
Drain crushed pineapple and reserved juice. You should get about 2/3 cup of juice which you can use in the cake (as below) or drink.Prepare cake as directed on box or use your favorite scratch recipe for yellow cake or pineapple cake. My grandmother used a box yellow cake, but instead of using water as directed, she used half pineapple juice/half water instead of all water.Set a large (3 quart) saucepan over medium heat and add butter. When butter is about halfway melted, add evaporated milk and sugar. Continue cooking over medium heat, whisking often, until butter is completely melted and mixture is hot and just beginning to simmer around the edges.
Add egg yolks and continue cooking over medium, whisking constantly, until mixture boils and thickens. Cook and whisk for a little over a minute, then remove from heat and let cool for about 5 minutes.
Add vanilla extract and drained pineapple. Leave pineapple mixture at room temperature (still in pan) while cake continues to bake and cools.When cake is cool, pour room-temperature pineapple mixture over cake.
Place in refrigerator and chill until pineapple topping sets.
