Preheat oven to 350°F. Line a sheet pan with parchment paper.
In the bowl of a stand mixer fitted with a paddle attachment, beat the butter and sugar on medium high speed until light and fluffy, about 3 minutes. Add the zest, vanilla, and salt and mix until combined.
Add the egg yolks one at time, mixing after each. Add the flour one cup at a time, mixing after each until just combined.
Roll a tablespoon of dough into a ball, then roll the ball into a long rope shape about 6 inches long. Shape into an "S" shape and place on the prepared cookie sheet. Repeat with remaining dough.
Bake 12-15 minutes, or until cookies just begin to turn golden on the edges. Do not over bake. Cool on a wire rack.
Cookies will keep in an airtight container for several days.
Calories: 107kcal, Carbohydrates: 12g, Protein: 2g, Fat: 5g, Saturated Fat: 3g, Cholesterol: 59mg, Sodium: 51mg, Potassium: 22mg, Fiber: 1g, Sugar: 1g, Vitamin A: 183IU, Calcium: 9mg, Iron: 1mg