2 sticks (1 cup) unsalted butter, room temperature
2/3 cup confectioners' sugar, sifted
2 large egg yolks, room temperature
1 teaspoon salt
2 teaspoons pure vanilla extract
Grated zest of 1 to 1 1/2 lemons
2 cups all-purpose flour
Approximately 1/2 cup granulated sugar, for coating
Place the butter in the bowl of an electric mixer fitted with the paddle attachment and beat on medium until smooth. Add the confectioners' sugar and beat until smooth and silky. Beat in 1 egg yolk, then the salt, vanilla, and lemon zest. Reduce mixer speed to low and add the flour, beating just until incorporated. Do not over-beat. Turn the dough out onto a work surface. Divide dough in two and wrap each piece in plastic wrap. Refrigerate for 30 minutes.
Working on a smooth surface, form each piece of dough into a log 1 to 1 1 /4 inches in diameter. Wrap logs in plastic wrap and refrigerate for 2 hours. Can be prepared up to this point and kept, well wrapped in the refrigerator for up to 3 days or frozen for up to 1 month.
Place one rack in the upper third and one in the lower third of the oven. Preheat oven to 350 degrees. Line two baking sheets with parchment paper; set aside.
Whisk the remaining egg yolk in a small bowl until smooth. Spread sugar out on a piece of wax paper. Remove the logs from refrigerator, unwrap, and brush lightly with the egg yolk. Roll logs in the sugar, pressing to adhere. Slice crosswise 1/4-inch-thick using a sharp knife. Place on prepared baking sheet, leaving about 1/2-inch between cookies.
Bake until set but not brown, 12 to 14 minutes. Transfer cookies to cooling rack until room temperature. Will keep stored in an airtight container, at room temperature, for up to 5 days.