Grease casserole dish with 1 tablespoon of unsalted butter. Save remainder to dot top of cake.
Dump pineapples with juice into a 9×13 buttered baking dish. Stir in vanilla extract and lemon juice
Dump (or sprinkle) cake mix evenly over fruit mixture in baking dish. Dot all over with pieces of cold unsalted butter (1 stick). Do not use margarine.
Bake 20 minutes. Carefully remove HOT baking dish from HOT oven. Use back of spoon to lightly spread melted butter evenly over cake mix until all of mix is coated with butter. Do this so entire top crust will be buttery (no dry cake crumbs).
Return to oven. Bake 20-25 more minutes or until bubbly and top cake crust is golden brown.
Serve warm or cold.
Top with vanilla ice cream or real whipped cream (optional).
NOTE: Refrigerate 2 hours up to overnight before serving cold.