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For filling, whisk sugar, water, and cornstarch in a heavy-bottomed saucepot.
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Whisk in egg yolks and salt and cook over low heat for 5 minutes, whisking constantly.
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Increase heat to medium and, still whisking, cook until filling becomes glossy and thick, about 5 more minutes.
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Remove from heat and strain.
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Stir in lemon zest, lemon juice and butter until butter dissolves.
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Pour immediately into cooled pie shell and let cool 15 minutes.
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Chill completely before finishing with meringue, about 4 hours.
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For meringue, preheat oven to 350 °F.
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Whip egg whites with cream of tartar until foamy.
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While whipping, gradually pour in sugar and whip on one speed less than highest until whites hold a stiff peak.
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Whisk in cornstarch and dollop over chilled lemon filling.
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Use sweeping motions with your spatula to create swirls and peaks.
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Bake pie for 10 minutes, just until meringue browns lightly.
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Let pie cool or chill until ready to slice.