Lemon juice (omit if you are using Jonathan or Kogyoku apples)
Soft light brown sugar
Bring the butter and eggs for the batter to room temperature.
Make the caramelized apples: Peel and core the apples and cut into 16 wedges.
Put the apple slices in a heatproof container, sprinkle with lemon juice, and cover with plastic wrap. Microwave for 6 minutes at 500W, stirring the apples every 2 minutes.
Mix the granulated sugar and water together in a frying pan and heat on medium.
Cook while stirring occasionally with a wooden spatula until the sugar becomes caramale colored. Once it starts to color, it changes very quickly so be careful.
Add the apple slices with the juices (the caramel will splatter, so be careful). Add the rum also. Simmer over medium heat while making sure the caramel coats the apple slices evenly.
Mix occasionally so that the liquid coats the apples, until it's reduced to about 1 to 2 tablespoons or so.
Butter the cake pan, pour the liquid in first, then line the bottom with the apple slices. See steps 9 and 10 for how to layer the apple slices.
Combine 3 slices of apple and put in the middle. Place 6 slices around that, layering them on each other.
Keep layering the apple slices, overlapping them slightly. See the "how to layer the apple slices" instructions below.
Make the batter. Add the soft brown sugar to the softened butter, and cream together well.
Add the beaten eggs in 3 batches, mixing well each time so that the batter doesn't split between additions. Add the milk and mix well so that the batter doesn't split.
Sift the cake flour and baking powder together as you add them to the batter, and mix in completely.
Pour the batter into the cake pan over the apples, and level it out with a spatula. If you tap the pan (the usual method for leveling out cake batter) the layered apple slices will shift around.
Bake in a preheated 170°C oven for 35 to 40 minutes. If a skewer inserted into the middle comes out clean, it's done.
Leave to cool in the cake tin. When it has cooled down a bit, place a board on top and flip it over to remove from the tin. Leave as is until the caramel has solidified.
Layer the apple slices, like this so that the top part of each slice will be on the surface when you turn the cake upside down.
This way of layering them is easier, but it doesn't look as nice since the core part of the top half of each slice will be hidden.
Story Behind this Recipe
This is a cake I came up with after getting a request from a former high school classmate for a Christmas cake. Her husband wanted something else, and she wanted an apple cake, so following the rules of society I went with her wishes. But the caramel flavor was requested by her husband.
When you put the apples in the caramel, it cools down rapidly and hardens, but as the apple is simmered it melts, so it's not a problem. By microwaving the apples first, when simmering them with the caramel, they will cook faster. Boiling and making the apples soft will make it easier to layer them as shown in the photos.