In a mixing bowl, combine all-purpose flour, granulated sugar, and salt, mixing until well blended. Cut in the unsalted butter until the mixture resembles coarse crumbs. Add the egg yolk and cold water, stirring until a dough forms. Press the dough into a tart or square pan evenly and chill it in the refrigerator for 20-30 minutes.

Preheat your oven to 350°F (175°C). Take the chilled crust out and prick it gently with a fork to prevent bubbling. Bake the crust for 12-15 minutes until lightly golden. Remove it from the oven and let it cool completely before adding the custard.

In a medium saucepan, whisk together granulated sugar, cornstarch, and lemon zest. Gradually add the whole milk, whisking until smooth. Cook over medium heat, stirring constantly until thickened and bubbling (5-7 minutes). Remove from heat and stir in fresh lemon juice, unsalted butter, and vanilla extract.

Pour the lemon custard into the cooled pre-baked crust, spreading it evenly. Bake for 25-30 minutes until the custard is set with a slight jiggle. Remove from oven and cool at room temperature.

Refrigerate the cooled cake for at least 4 hours, preferably overnight, to set the custard and enhance flavors. Dust with powdered sugar before serving.