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1
Combine flour, sugar, and salt in processor. Add butter. Using on/off turns, cut in butter until coarse meal forms. Add yolks and process until moist clumps form. Gather dough into ball. Divide into 6 equal portions. Press 1 dough portion over bottom and up sides of 4 1/4- to 4 1/2-inch-diameter tartlet pan with 3/4-inch-high sides and removable bottom. Repeat with remaining dough, forming 6 tartlets total. Place on rimmed baking sheet, cover with plastic, and chill 1 hour. (Can be made 1 day ahead. Keep refrigerated.)
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2
Preheat oven to 375u00b0F. Bake crusts until golden, pressing with back of fork if crusts bubble, about 20 minutes. Cool.
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3
Whisk eggs, yolks, 3/4 cup sugar, and lemon peel in medium metal bowl. Place bowl over saucepan of barely simmering water; whisk until sugar dissolves, about 1 minute. Stir in lemon and lime juices. Whisk until mixture thickens, about 4 minutes. Remove from heat; add butter in 3 additions, whisking until smooth. Strain lemon curd through fine sieve into medium bowl. Press plastic directly onto surface of curd. Chill until cold, at least 5 hours.
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4
Preheat oven to 425u00b0F. Place 1 1/4 cups lemon curd in large bowl; reserve. Divide remaining curd among tartlet crusts, spreading evenly over bottoms.
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5
Using electric mixer, beat egg whites in large bowl until frothy. Add salt and beat until soft peaks form. Gradually add remaining 6 tablespoons sugar, beating until stiff but not dry. Fold half of egg whites into reserved lemon curd. Gently fold in remaining whites. Divide egg-white mixture among tartlets, sealing to crust edges and mounding in center. Bake until filling is golden brown and set, about 13 minutes. Let tartlets stand at room temperature 5 minutes. Push pan bottoms up, releasing tartlets. Transfer tartlets to plates. Dust tartlets with powdered sugar and serve warm or let stand up to 30 minutes.
