Place a fine mesh sieve or a sifter over a medium mixing bowl and add flour and salt. Sift into bowl and set aside.
In a mixing bowl, combine butter and sugar; using a handheld mixer on medium-high speed, cream butter and sugar together until mixture is light in color and has a fluffy texture. Scrape down sides of bowl with a rubber spatula, then add vanilla extract, lemon zest, and egg. Beat until blended, 1 or 2 minutes longer. On low speed, gradually add sifted flour and salt, mixing until just blended.
Remove the dough from bowl and divide into 2 balls. Place each portion of dough between 2 pieces of waxed or parchment paper and roll into 5-inch-diameter disks. Place dough on baking sheets and refrigerate until firm, at least 2 hours or up to 24 hours.
When you are ready to roll and bake cookies, preheat oven to 350 degrees. Line a baking sheet with silicone liners or parchment paper. Place 1 disk of cookie dough onto a work surface and remove top sheet of paper. Lightly dust surface of dough with flour. Starting from center, roll out dough to an even thickness of 1/8 inch. Using decorative cookie cutters, cut dough directly on parchment paper. Use a spatula to transfer dough to lined baking sheet. Place cookies at least 1/2 inch apart.
Gather dough scraps and reroll them to make additional cookies. Only reroll dough once.
Bake cookies 1 sheet at a time for 10 to 12 minutes or until cookies are golden around edges and firm to the touch. Transfer cookies to a wire rack to cool. To frost cookies, prepare a recipe of Royal Icing and decorate as desired.
Cook's Note: The cookies can be stored in resealable plastic containers or cookie tins for up to 1 week.