Preheat oven to 350
Cover bottom of 10"x15" Glass Pan with Melted Butter*. (I usually slice the butter & put it in the pan then place the pan in the oven just long enough for it to melt while it's preheating. Saves a dirty dish.)
Sprinkle Brown Sugar & Coconut over Melted Butter. Lay pineapple slices on top. (Do NOT discard Pineapple Juice)
Place Dry Cake Mix in large bowl or stand mixer. Add Eggs & Oil per package instructions. (If I use a White Cake Mix, I don't use egg whites - I use the whole egg for a richer flavor)
You aren't going to use Water in your Cake Mix but you are going to use the same amount of liquid. In your measuring cup, add reserved Pineapple Juice, Coconut Extract/Rum. If you are still a little short of the amount of liquid you need, use water to reach that level. (I've found most cans of pineapple have almost enough liquid, but just not quite.)
Beat cake mix according to instructions on package. When done, evenly spread over Pineapple/Coconut Mixture in your pan.
Bake 30-35 minutes until a toothpick inserted in the middle comes out clean.
As soon as it comes out of oven run a butter knife around the edges to loosen any stuck spots. Place a large cookie sheet over top of cake pan and turn the whole thing upside down. Let sit for a couple of minutes until cake and ooey-ness falls onto cookie sheet. Using a rubber spatula, scrape the rest of the caramelized coconut out of the pan and spread into spots that look like they could use a touch more. Let cool and serve with ice cream or whipped cream.
*If you prefer, you can make individual cakes too. In the photo above, I used Mini Bundt Pans and put my coconut in the center hole of the pineapple slice. Large Muffin Tins would work too or use regular muffin tins and crushed pineapple.