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Preheat oven to 400°.
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Line two baking sheets with baking parchment and set aside.
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Beat egg whites and salt in small bowl at high electric-mixer speed until frothy.
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With mixer still at high speed, add sugar gradually and continue beating until meringue peaks stiffly (about 4 minutes).
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Beat in vanilla, then by hand fold in lemon zest.
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Drop meringue from rounded teaspoon onto prepared baking sheets, spacing 2-inches apart.
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Set in oven, immediately turn oven off, and let cookies remain in the oven for 2 hours (you can leave them in overnight) Peel meringue off parchment and store in airtight container.