In a large mixing bowl, beat unsalted butter with granulated sugar until creamy and smooth, about 2-3 minutes. Add salt and pure vanilla extract, mixing until well combined. Gently incorporate the egg yolk, followed by all-purpose flour. Mix until just combined.

Flour your work surface lightly, then turn the dough out onto it. Knead the dough gently just until it becomes smooth, about 1-2 minutes. Shape the dough into a log about 2 inches in diameter, wrap tightly in plastic wrap, and refrigerate for at least 2 hours.

Preheat your oven to 325°F (160°C) and line two baking sheets with parchment paper. Once chilled, slice the dough into ⅛-inch thick rounds and arrange on baking sheets, allowing space for even baking.

Bake for 16-18 minutes, or until the edges are just golden. Remove from the oven and let cool on the sheets for 5 minutes before transferring to a wire rack.