Peel the apples and cut into very small cubes. In a pan over medium heat, heat the butter, sugar, and lemon juice together. Add the apples and simmer for 3-4 minutes, then add a little cinnamon. Drain through a sieve and reserve the apple stock.
Using a cappuccino cup, cut 8 circles from the puff pastry. Place each in a greased muffin case and refrigerate.
In a saucepan over medium heat, heat the milk with the vanilla pulp and vanilla pod, bringing to a boil. Remove from heat and stir in the flour, a pinch of salt, and 150g sugar. Return to medium heat and bring to a boil again. Remove from heat, discard the vanilla pod, and stir in the egg and egg yolk until creamy. If needed, briefly return to heat while stirring to reach the desired consistency.
Fill each pastry case with the apples. Cover with the vanilla cream. Bake at 225°C (437°F) with top and bottom heat for 10 minutes. Remove from oven. Mix the almond flakes with the reserved apple stock and spread over the natas. Return to oven and bake for another 8 minutes. Allow to cool before serving, dusted with powdered sugar.