Preheat oven to 350F degrees. Lightly spray a 9x13 pan with cooking spray and then line the bottom of it with pineapple slices.
Mix together yellow cake mix according to package instructions and pour the batter over the pineapple slices.
Bake the cake according to boxed cake directions. Cake is finished when a toothpick inserted into the middle of the cake comes out clean.
Allow the cake to cool completely.
Poke holes into cake using the end of a wooden spoon. You want about 12-16 holes.
In a bowl, whisk together pudding with milk until smooth. Pour the mixture onto the cake and spread out with a knife or spatula, gently pushing down to make sure the pudding gets down into the holes.
Top with whipped topping and pineapple chunks and then refrigerate for a few hours prior to serving. This cake gets better the longer it sits so you can make it the day before you need it. Just keep it in the fridge.