Salted butter is the key ingredient in these popular French cookies, which is no surprise, as they hail from Brittany, the region famous for its butter.
Pulse butter and sugar in a food processor until well combined and smooth. Add 2 egg yolks and pulse to combine. Add flour and pulse until dough comes together. On a lightly floured surface, shape into a disk, wrap in plastic, and refrigerate at least 2 hours or up to overnight.
Preheat oven to 350 degrees with rack in top third. On a lightly floured surface, roll out dough to 1/8 inch thick. Using a 2 1/2-inch fluted round cutter, cut out rounds; transfer to parchment-lined baking sheets, spacing 2 inches apart. Reroll scraps once and cut out more rounds, if desired. Using the tines of a fork, press to create a crosshatch pattern. Refrigerate until firm, about 30 minutes.
In a small bowl, whisk together remaining egg yolk and 1 teaspoon water; use to brush tops and sides of cookies. Bake, rotating sheets halfway through, until golden brown, about 15 minutes. Transfer sheets to wire racks; let cool completely.