Preheat the oven to 190°C. Grease a 15-16 cm cake tin, or line a standard muffin tin with 6 paper cases.
Put 50 g of the sugar into a small bowl, add the cinnamon and stir together.
Sift the flour and baking powder together.
Cream the butter with the remaining 85 g sugar. Beat the egg and vanilla (if using) with a fork, then beat into the creamed mixture.
Add 1/3 of the flour & baking powder mix, then follow with 1/2 the milk and repeat, then fold in the last 1/3 of the flour & baking powder.
Peel the apple and cut into thin slices and cut the slices in half. Toss in a little lemon juice.
Spread half the cake mixture in the tin. Lay the apples on top.
Scatter half the cinnamon sugar over the top. Spread the rest of cake mixture on top of the apples (it doesnt matter if there are gaps). Scatter the remaining cinnamon sugar on top.
Bake for 25 minutes (or 20 minutes if making individual ones in a muffin tin).
Leave to cool in the tin for at least half an hour before turning out carefully, as it is a very soft cake.
Also really good with the addition of a handful of blackberries on top of the apples. I am sure blueberries would be nice too.
You can use an eating apple but the texture isn't quite as squidgy and you may want to reduce the amount of cinnamon sugar on the apples by 1/4 to 1/3.
Double the quantities for a 20-23 cm cake tin.