Preheat the oven to 320°F (160°C) using top and bottom heat.
In a large mixing bowl, combine the self-rising flour, butter, light brown sugar, eggs, vanilla sugar, and a generous pinch of salt. Mix until you have a smooth cake batter.
Line an 8-inch/20 cm loaf pan with parchment paper, or grease it with butter and lightly dust it with flour. Pour the batter into the pan and smooth the top with a spatula.
Wash the apple and remove the core. Cut it into thin slices or half-moon slices. A mandoline works well for getting them nice and thin.
Place the apple slices in a bowl and add the lemon juice, ½ tablespoon sugar, and cinnamon. Toss until all the apple slices are lightly coated with cinnamon sugar.
Insert the apple slices upright into the batter, with the peel facing upward. Continue until the top of the cake is covered with apple slices.
Bake for about 45 minutes. Check for doneness by inserting a toothpick or cake tester into the center of the cake. If it comes out clean, the cake is done. If there is still wet batter on it, bake for a few minutes longer.
Let the apple cake cool slightly before slicing. Enjoy!