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["Whisk flour and salt in a medium bowl. Using an electric mixer on medium-high speed, beat butter, sugar, lemon zest, and vanilla in a large bowl, occasionally scraping down sides, until light and fluffy, about 3 minutes. Add egg yolks; beat just to blend. Reduce speed to low; add flour mixture and beat, occasionally scraping down sides, just to blend. Divide dough in half; roll each half into a 10""-long log about 1 3/4"" in diameter. Wrap in plastic and chill until firm, about 1 hour. DO AHEAD:", "Arrange racks in lower and upper thirds of oven; preheat to 350u00b0F. Line 2 baking sheets with parchment paper. Unwrap 1 dough log. Using a sharp, lightly floured knife, cut log into 1/4""-thick rounds. Transfer to prepared sheets, spacing 1"" apart.", "Bake until cookies are firm and golden brown around edges, 16-18 minutes. Let cool for 1 minute, then transfer to wire racks and let cool completely. Repeat with remaining dough log, using cooled baking sheets and new parchment paper.", "Whisk sugar and 2 tablespoons juice in a small bowl, adding more juice by 1/2-teaspoonfuls if too thick. Add coloring, if desired. Spread or drizzle icing over cookies. Decorate as desired. Let stand until icing sets, about 10 minutes. DO AHEAD:"]
