Cream the softened unsalted butter and granulated sugar in a mixing bowl until smooth and creamy, about 2–3 minutes.

Incorporate the egg yolk and pure vanilla extract, beating on low speed until fully combined, about 1 minute.

Sift in the all-purpose flour and salt, mixing gently until just incorporated—do not overmix.

Shape the dough into a log, wrap in plastic wrap, and refrigerate for at least 2 hours.

Preheat the oven to 325°F (160°C) and line baking sheets with parchment paper.

Slice the chilled dough into ⅛-inch thick rounds and place them 1 inch apart on prepared sheets.

Bake for 16–18 minutes until edges are just golden brown, then let cool on sheets for 5 minutes.

Transfer cookies to a wire rack to cool completely.