Peel the apples and cut them into very small cubes. Heat the butter, sugar, and lemon juice in a pan over medium heat. Add the apple cubes and simmer for 3-4 minutes. Add a pinch of cinnamon. Drain the apples through a sieve and save the liquid (apple stock).
Cut eight circles from the puff pastry using a cappuccino cup. Place the pastry circles in greased muffin molds and refrigerate for roughly 15-20 minutes until chilled.
Heat the milk in a saucepan with the vanilla pod and vanilla pulp over medium-high heat, bringing to a boil. Remove from heat and stir in the flour, a pinch of salt, and sugar. Return to medium-high heat and bring to a boil again. Remove from heat and discard the vanilla pod. Stir in the egg and egg yolk. Return to the heat and bring to a boil while stirring continuously until the mixture reaches a creamy consistency.
Place the apples on the bottom of each puff pastry circle and cover with the vanilla cream. Bake at 225°C (437°F) using top and bottom heat for 10 minutes. Remove the muffin tin from the oven. Mix the almond flakes with the reserved apple stock and spread over the natas. Return the tray to the oven and bake at 225°C (437°F) for another 8 minutes. Allow to cool and serve dusted with powdered sugar.