Preheat grill to medium heat. Slice and cut pineapple into rings, coring the center of each pineapple ring. Brush grill with oil and grill the pineapple until tender and grill marks are made. (Approximately 2-3 minutes). Flip and cook pineapple on other side. Remove from grill and set aside.
Preheat oven to 350 degrees. Lightly grease a 9x13 inch pan or two pie pans and set aside.
In a bowl, mix together the melted butter and sugar. Pour into the bottom of your lightly greased cake pan. (You can use substitutes like Swerve if desired for your sugar. If you don't like a super sweet cake, omit the sugar and just use the butter).
Pour the melted butter or melted butter/sugar combination into the bottom of the cake pan. Layer the pineapples evenly in the pan.
Prepare the cake mix according to package directions. Pour evenly over the top of the pineapple. Cook at 350 degrees for 25-30 minutes or until the cake is nicely browned and a toothpick inserted into the center of the cake comes out clean.
Remove from oven and allow cake to cool for approximately 20 minutes before inverting onto platter. Allow to cool completely then top with lime zest and toasted coconut flakes if desired. Slice and top with whipped topping.
Notes
The cake is best served fresh. Cake can be stored for up to three months in the freezer. Thaw to serve.