For the Caramel:

1. Preheat your oven to 100°C (210°F) and place a 22x11cm (8.6×4.3in) mould inside to warm.
2. Spread a thin layer of sugar in a pan over medium heat. As it melts, add more sugar to the melted spots until all the sugar is used.
3. Tilt the pan to mix the caramel without using utensils (to prevent crystallization).
4. Once the caramel turns a golden amber color, carefully pour it into the warm mould, rotating to coat the base and sides. Set aside.

For the Apple Millefeuille:

1. Peel and thinly slice the apples (1mm/0.03in thick) using a mandolin. Slice only half the apples initially, reserving the rest as needed.
2. Mix the sugar, salt, and cornstarch in a bowl.
3. Layer the apple slices in the caramel-coated mould, sprinkling the sugar mixture between each layer.
4. Press down firmly every 4 layers to compact the apples.
5. Bake at 180°C (350°F) for 1 hour. Place the mould on a baking sheet to catch any drips.
6. Once baked, let it cool slightly. Run a knife around the edges, place a serving dish on top, and flip to unmould.

For the Vanilla Custard:

1. Whisk the egg yolks and cornstarch in a bowl.
2. In a pot, heat the milk, vanilla, sugar, and salt until steaming.
3. Gradually add a bit of the hot milk to the egg mixture, whisking constantly. Then, pour the egg mixture back into the pot.
4. Cook over medium-low heat, stirring continuously, until the custard thickens (do not boil).
5. Remove from heat and transfer to a clean bowl to stop the cooking process.

Serving:

Slice the apple millefeuille and serve with a generous drizzle of vanilla custard. Enjoy warm or at room temperature!