Shortbread Swirls

Preheat oven to 350 degrees. Whisk together flour and salt. In a large bowl, beat butter with vanilla seeds and confectioners' sugar on medium-high speed until pale and fluffy, about 2 minutes. Add yolk and beat until incorporated, 1 minute more. Gradually add flour mixture and beat on low until just combined. Tint with food color as desired.

Transfer dough to pastry bag fitted with a 1/2-inch star tip (such as Ateco #825). Pipe into S shapes, each about 3 inches long and 1 inch wide, spaced 2 inches apart on ungreased baking sheets. Freeze until firm, about 20 minutes.

Bake until edges are light golden, 16 to 18 minutes. Transfer cookies on sheets to wire racks; let cool completely. Cookies can be stored in an airtight container at room temperature up to 3 days.