In a medium bowl mix flour and salt. Set aside.: The first scent you notice is the dry, slightly nutty aroma of all purpose flour combined with the faint sharpness of salt , which seems subtle but plays a major role in flavor balance. Mixing them first ensures even distribution, so when the wet ingredients are incorporated you avoid pockets of unseasoned flour that can create dry spots. A common mistake is rushing this step and adding the flour too quickly, which can lead to overmixing later when you try to incorporate it. Keep the bowl close at hand, and set this mixture aside so the next steps flow smoothly.
In a large bowl, using an electric mixer beat butter and granulated sugar on medium speed until light and fluffy.: You will hear the soft whir of the mixer and see the butter turn paler as tiny air bubbles become suspended; this aeration is what gives the cookies that tender, slightly cakey interior. The texture should be noticeably lighter and the smell richer, like sweet cream. Resist the urge to crank the speed too high, which can warm the butter and make the mixture greasy. If your kitchen is warm, pause and let the bowl rest so the mixture stays stable.
Add egg, egg yolk, lemon zest, lemon juice, lemon and vanilla extract, and a few drops of food coloring, if using. Mix well until combined.: The bowl releases a bright citrus perfume as you add the egg , lemon zest , and lemon juice . Stirring brings a glossy sheen as the components emulsify, and the aroma becomes more pronounced when the extracts are added. Because these ingredients are responsible for flavor and structure, mixing until just blended avoids tough cookies caused by overworked gluten. One mistake people make is adding cold eggs straight from the fridge; room temperature eggs integrate more smoothly and produce a uniform texture.
Gradually beat in on low speed the flour mixture until just combined. Divide the batter in half and shape each half into a log about 9 inches long and 1 1/2-2 inches in diameter. Wrap in wax paper and refrigerate for 1 hour or until firm.: As the dry mixture disappears into the wet, you will see the dough transform from sticky to cohesive, and the sound of the mixer will soften. Working on low speed preserves tenderness by minimizing gluten development. Once divided and rolled into logs, the dough feels cool and pliable; chilling firms the fat, which helps the cookies hold their shape and slice cleanly. A common pitfall is under chilling, which leads to misshapen cookies, or over chilling, which makes them hard to slice. Aim for a firm, but not rock solid, texture.
Preheat oven to 350°F.: The oven gives off a dry warmth that prepares the baking environment; setting it early ensures it reaches 350°F so the cookies start baking at the right temperature. A properly preheated oven promotes even spread and browning at the edges. If you put the cookies into an oven that is still heating, they may spread incorrectly and lose that delicate edge crispness, so give it time to reach temperature before baking.
Slice the dough into 1/4-inch thick slices and arrange on ungreased baking sheets covered with parchment paper.: When you slice the chilled logs, the knife should glide through and produce neat discs with clean edges. You may hear a faint snap as the chilled dough yields, and the raw disks will feel cool and slightly firm. Placing them on parchment prevents sticking and facilitates easy transfer to racks. A frequent error is slicing too thick or too thin; too thick yields underbaked centers, too thin produces brittle cookies, so aim for consistent 1/4 inch slices.
Bake for 12 to 15 minutes or until very lightly browned on the edges.: During baking you will notice a tender cookie taking shape and a faint scent of baked butter and lemon . The edges should develop a whisper of golden color while the centers remain pale, which signals doneness. Baking longer than recommended will dry them out and reduce the soft interior, so keep an eye through the oven window starting at 12 minutes. If your oven runs hot, err toward the lower end of the time range to preserve tenderness.
Cool on baking sheets for 1 minute, after that transfer the cookies to wire racks to cool completely.: The cookies will release a warm, toasty aroma as they rest. Letting them sit briefly allows the structure to set so they do not break when moved. Transferring to a wire rack lets air circulate, preventing sogginess on the bottom and helping the cookies reach room temperature for glazing. A common mistake is leaving them on the baking sheet too long, which can cause residual heat to overbake the bottoms.
Mix the powdered sugar and lemon juice in a medium bowl. Stir until smooth. For a thinner glaze, add more lemon juice, a teaspoon at a time. For a thicker glaze, add more powdered sugar, a tablespoon at a time.: When you stir the powdered sugar into the lemon juice , the mixture transforms into a glossy, opaque glaze with a bright, citrus scent. Start with small additions of liquid or sugar to reach your desired viscosity; the sheen will be shiny and the aroma tart. Over thinning will make the glaze run off the cookies, while over thickening yields clumpy, matte results. Adjust very gradually to control consistency.
Dip the bottom (or top if desired) of each cookie into the glaze, then gently set the cookie with the glaze side up, onto a wire cooling rack to allow the glaze to set up.: Dipping gives you a smooth coating that sets with a faint crackle. As the glaze dries you may hear a barely audible tack as it loses moisture, and the surface will matte slightly while retaining a pleasant gloss. Allowing them to set on a rack avoids pooling and keeps the bottom crisp. One mistake is glazing too soon while cookies are warm, which causes the glaze to thin and run, so ensure cookies are fully cooled before dipping.
If desired top with lemon zest immediately, for it to stick.: The final flourish of fresh lemon zest adds aroma and visual contrast, and pressing it in while the glaze is tacky helps it adhere. The tiny citrus oils will lift the scent, enhancing the first inhale before a bite. If you wait until the glaze is fully hard, the zest will not stick well and will fall off during handling, so apply right away for best results.