Preparation

Mix the dough:

• Sift the flour and salt into a medium bowl. In the bowl of a stand mixer fitted with the paddle attachment, beat the butter and both zests on medium low until well blended, about 2 minutes. Add 1 cup of the sugar in a steady stream and mix for another 2  minutes until well blended. Blend in the lemon juice, lime juice, and vanilla. Reduce the speed to low and add the flour mixture in two additions, mixing just until blended.

Shape the dough:

• Have ready two 15-inch sheets of plastic wrap. Put the remaining 1/4 cup sugar in a long, shallow pan (like a 7×11-inch Pyrex dish). Shape one half of the dough into a log about 10  inches long and roll gently in the sugar to thoroughly coat. Posiition the log on a sheet of plastic, centering it on the long edge closest to you. Roll tightly, twisting the ends of the plastic firmly to seal. With your hands on either end of the log, push it firmly toward the center to compact the dough. The finished log should be about 9 inches long and 1-1/2  inches thick. Repeat with the remaining dough. Refrigerate the logs until firm enough to slice, about 2-1/2  hours, or freeze for up to three months.

Bake the cookies:

• Position a  rack in the center of the oven and heat the oven to 375°F. Line  two rimmed baking sheets with parchment.
• Working with one log at a time, use a sharp, thin-bladed knife to cut 1/8-inch thick rounds. Set the rounds 1 inch apart on the baking sheets and bake one sheet at a time until lightly browned around the edges, about 10 min., rotating the sheet as needed for even browning. Let cool on the pan for about 5 minutes before transferring the cookies to racks with a thin-bladed spatula. When cool, store layered between sheets of waxed paper in airtight containers for up to a week, or freeze for up to three months.