Plump the raisins in warm water (and rum extract if you like).Important!: Do NOT use a usual bottom detachable cake pan as caramel can drip down into the oven.Make the caramel into a round sturdy pan on low - medium flame and add butter to finish. Coat the walls of the pan as well as you can. Prepare the apples: peel them and discard their core. Place them one by one into the pan making sure they stand well one next to each other. Drain the raisin water and fill the apples with raisins and chopped walnuts. Add a little fruit jam as well if you prefer. Bake at 350 F for about 30 minutes.
10 minutes before taking out the apples, start making the sponge batter. Beat the egg whites with a pinch of salt until really fluffy. Start adding the sugar and mix until the sugar dissolves. Add the yolks, vanilla extract and the orange zest. Mix a little more to blend everything together.Mix the baking powder and flour and start adding it in batches into the batter with a hand mixer, folding it in slowly. Take out the apples from the oven and sprinkle the cinnamon over them. Then pour over the batter. Level with a spatula and turn back to the oven for another 1/2 hour or until a toothpick comes out clean.
Let it cool for 5 minutes. Take a silicone spatula and go around the inside rim to ensure the cake does not stick to the pan. Place a large platter over the pan and with a quick move reverse it. Carefully remove the baking pan and let the apple cake cool completely.Serve with ice cream and /or whipped cream on top for the most delicious apple treat ever.