Preheat your oven to 375°F (190°C). Generously butter a 9-inch tart pan and roll out the puff pastry until it's about 1/8 inch thick. Press the dough into the pan, trim excess, poke holes in the bottom, and chill in the refrigerator.

In a medium saucepan, combine 1 cup of whole milk and 1 cup of heavy cream over medium heat. In a mixing bowl, whisk together 3 eggs, 3/4 cup of granulated sugar, 2 tablespoons of cornstarch, and 1 teaspoon of vanilla extract until smooth.

Gradually pour a third of the warm milk mixture into the egg mixture, whisking constantly to temper the eggs. Return to the saucepan and cook over medium heat, stirring continuously for about 5 minutes until the custard thickens.

Remove the chilled pastry shell and layer the thinly sliced apples evenly across the bottom. Pour the thickened custard mixture over the apples, smoothing the top with a spatula.

Bake in the preheated oven for 40-45 minutes until the top is golden and slightly puffed, and the center jiggles slightly. Cool for 30 minutes before refrigerating for at least 2 hours.

Dust the top with powdered sugar and serve with optional apricot jam or lemon zest for an elegant touch.