For Perfect Upside-Down Cake, Let Gravity Do Most Of The Work
Baking a pineapple upside-down cake, or really any upside-down cake in general, seems simple at first glance, but any baker will let you know it's more difficult than it looks. Reddit is full of horror stories of upside-down cakes ending up under-baked or moist messes. Likewise, perhaps, your cake is sticking to the pan after you flipped it.
When it comes to baking, experienced bakers will tell you that patience is key. Prior to flipping, you want to make sure that your cake is actually done. Golden brown with dark edges is characteristic of a perfectly cooked upside-down cake. Of course, when in doubt, use a cake tester. When it comes to flipping, use a knife to free the edges of the cake from the pan, then flip the cake upside down and wait.
According to Completely Delicious, you need to wait several minutes after flipping the cake before removing the pan. Let it sit there, completely cooling off, while gravity does most of the work. Then, slowly lift the pan being careful of the fruit at the bottom.
Wait for your upside-down cake to cool, but not that long
While glazed caramel may be the best part of an upside-down cake, it can also make your cake stick to the pan. You want to avoid letting your cake sit too long once out of the oven, as this will make the caramel harden. Harden caramel can make flipping your cake a disaster, with fruit sticking to the bottom of the pan. The rule of thumb is only to allow your cake to cool off for 10 minutes before removing the pan.
If your caramel mixture does harden, fear not — Cake Decorist suggests putting the pan over some hot water for five minutes so the caramel will melt. This should make it easier to remove the pan. If you are still not satisfied with the result, reconsider the pan you are using, or how you are greasing it before putting it into the oven. A non-stick cake pan or skillet should be perfect for this cake. However, you could also consider using a baking spray or making a homemade baking spray.
Besides patience and meticulosity, remember that practice makes perfect. For now, trust in gravity, and consider every unsuccessful recipe as a lesson learned.