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1
In the bowl of a stand mixer, cream the butter until it is smooth. Place a sifter over the bowl of the mixer, and sift 2/3 cup confectioners' sugar directly into the bowl. Mix until smooth, scraping down the sides of the bowl once or twice. Add egg yolk, salt, vanilla, and lemon zest. Mix well, again scraping the sides of the bowl once or twice. Add the flour and mix on low just until combined. Finish mixing with a spoon to avoid over-mixing.
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2
Divide the dough in half, wrap each half in plastic, and refrigerate for 30 minutes. This will help you be able to roll the dough into logs.
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3
After 30 minutes of chilling, unwrap the dough, leave it on the plastic wrap and form into a log about 1 - 1.5 inches in diameter. I usually re-wrap it in plastic and use the plastic to roll it into a log. This helps the dough not stick to my hands. Refrigerate the logs for at least 2 hours or freeze for at least 45 minutes. This allows them to firm up so you can slice them.
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4
Preheat the oven to 350 F. Whisk the egg yolk in a small bowl until smooth. Unwrap one of the dough logs, brush with egg yolk, then roll it in sugar so it's covered completely. Slice the log into slices that are about 1/4-inch thick and lay slices on a baking sheet lined with parchment or a baking mat. Place them about 1 inch apart. The cookies will expand slightly when baking.
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5
Bake for 12-14 minutes or until set but not browned. The edges will be slightly darker due to the egg yolk. Let them cool on the pan for 5-10 minutes, then remove to a wire rack to cool completely. Repeat the process with the second log of dough while the first batch cools.
