In a large mixing bowl, combine flour, salt, and sugar. Cut in unsalted butter until the mixture resembles coarse crumbs. Gradually add cold water until a dough forms. Divide, wrap, and chill for 30 minutes.

Preheat oven to 350°F (175°C). Roll out each dough portion into rectangles for baking sheets. Transfer to a parchment-lined baking sheet, prick with a fork, and bake for 25–30 minutes until golden. Cool completely.

In a saucepan, combine milk and sugar; heat until simmering. In a bowl, beat egg yolks and cornstarch, whisk in hot milk, then return to saucepan. Cook, whisking until thickened, about 5–7 minutes. Stir in vanilla extract and butter.

Spread custard over one pastry layer, place second layer on top, and refrigerate for at least 2 hours.

Dust with powdered sugar before serving and slice into squares or rectangles.