In a mixing bowl, combine all-purpose flour, granulated sugar, and salt, blending well. Cut in cold, cubed unsalted butter until the mixture resembles coarse crumbs. Mix in an egg yolk and enough cold water to form a dough. Chill in the refrigerator for 20–30 minutes.

Preheat your oven to 350°F (175°C). Roll out the chilled dough on a lightly floured surface and transfer it to the tart pan. Prick the bottom with a fork to prevent bubbling. Bake the crust for 12–15 minutes until lightly golden. Let cool slightly.

In a medium saucepan, whisk together sugar, cornstarch, and lemon zest. Gradually add egg yolks and whole milk while stirring. Cook over medium heat, stirring until it thickens enough to coat the back of a spoon, about 5–7 minutes. Stir in lemon juice, unsalted butter, and vanilla extract until smooth.

Pour the lemon custard into the pre-baked crust, spreading evenly. Bake for another 25–30 minutes until the custard is set but slightly jiggly in the center.

Remove from the oven, let cool completely, then refrigerate for at least 4 hours or overnight. Dust the top with powdered sugar before serving.