Step 1

In a large bowl, using an electric mixer, beat butter, sugar, and salt until light and fluffy. Add yolks, one at a time, scraping the bowl after each addition. Add vanilla. With mixer on low, add flour; mix just until combined. Wrap in plastic; chill until firm, about 1 hour.

Step 2

Citrus or Spice: Whisk 1 tablespoon grated lemon, lime, or orange zest (or a combination), or 1 tablespoon pumpkin-pie spice into the flour before adding it to the butter mixture.