Prepare the Pastry:

1. Combine the all-purpose flour, powdered sugar, and cubed unsalted butter in a large bowl. Use your fingertips to rub the butter into the flour until it resembles sand.
2. Add the egg yolk and mix until a shaggy dough forms.
3. Transfer the dough to a floured surface and knead until a ball forms.
4. Cover the dough with cling wrap and chill for 30 minutes, or until firm.
5. Grease egg tart molds with butter and lightly dust with flour.
6. Divide the chilled dough into 12 equal balls, approximately 30 g each.
7. Press the dough into the tart molds, extending the dough 5 mm above the edge of the mold to create a taller tart. Chill for 1 hour or until firm.

Prepare the Custard:

1. Heat the whole milk and granulated sugar in a small saucepan over medium heat for 5 minutes, or until the sugar has dissolved.
2. In a medium-sized heatproof bowl, combine and whisk the large eggs and vanilla extract until well combined.
3. Slowly pour the steaming milk into the egg mixture, whisking continuously until well combined.
4. Pour the custard through a fine-meshed sieve into another bowl or jug. Set aside until the tart pastry has completely chilled.

Baking:

1. Preheat the oven to 320°F (160°C).
2. Remove the chilled tart shells from the fridge and fill each shell to the top with the egg custard.
3. Bake the tarts for 35 minutes, or until the custard is just set. A toothpick inserted in the center of the custard should stand straight without falling.
4. Remove from the oven and cool for 10 minutes before unmolding. Egg tarts are easier to unmold when warm.