01 - In a food processor, pulse flour, powdered sugar, and salt. Add cold butter and pulse until the mixture resembles coarse crumbs. Add egg yolk and 2 tablespoons of cold water; pulse until dough comes together, adding more water if necessary.
02 - Transfer dough onto a lightly floured surface. Shape into a disk, wrap tightly in plastic wrap, and refrigerate for 30 minutes.
03 - Preheat oven to 350°F. Roll out dough to fit a 9-inch tart pan. Press dough into the pan and trim the edges. Prick the base with a fork.
04 - Line the shell with parchment paper and fill with pie weights or dried beans. Bake for 15 minutes. Remove weights and parchment, then bake for 10 more minutes until lightly golden. Allow to cool slightly.
05 - In a bowl, whisk together sugar, eggs, egg yolks, lemon zest, and lemon juice until fully combined and smooth.
06 - Pour mixture into a saucepan and cook over medium-low heat, stirring constantly, until thickened and able to coat the back of a spoon, about 8 to 10 minutes.
07 - Remove custard from heat. Whisk in softened butter pieces until the mixture is smooth and glossy.
08 - Pour the lemon filling into the cooled tart shell and smooth the surface with a spatula.
09 - Bake the filled tart for 10 to 12 minutes, until the custard is just set but still has a slight wobble in the center.
10 - Allow the tart to cool completely at room temperature, then refrigerate for at least 1 hour before serving.