What are the best apples for upside-down cake?
Pick apples that hold their shape when baked. Honeycrisp gives you sweet-tart flavor and clean slices. Granny Smith is firmer and more tart, which is awesome against caramel. Pink Lady is another great in-between option. Avoid very soft apples that turn to applesauce.
What pan size should I use?
This recipe is written for a 9-inch round cake pan (at least 2 inches deep). If you use an 8-inch pan, it will be thicker and take longer. A 10-inch pan makes it thinner and it can bake faster.
How long should I cool it before flipping?
Cool the cake in the pan for 15 minutes. That window is the sweet spot. The caramel is still warm enough to release, but it has started to set so the apples are less likely to slide.
How do I know the cake is done?
Look for three cues: the center should spring back lightly when you tap it, the edges should pull just a bit from the pan, and a toothpick inserted into the cake portion should come out clean or with a few moist crumbs. If you see wet batter, give it 3 to 5 more minutes. If you like thermometers, aim for about 200 to 205°F in the cake portion (not the apple layer).
Can I add nuts or extras?
Yes. Sprinkle 1/2 cup chopped toasted pecans or walnuts over the caramel before the apples, or tuck a small handful between apple slices. Keep it in an even layer so the flip stays neat.
Why did my apples stick to the pan?
Usually it is one of three things: the caramel cooked too long and got too hard, the cake cooled too long in the pan, or the pan was not greased well on the sides. Next time, flip at 15 minutes and run a thin knife around the edges first. For extra insurance, you can also line the bottom with parchment.