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1
Put oven racks in upper and lower thirds of oven and preheat oven to 200u00b0F.
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2
Using the bottom of the springform pan as a guide, draw 2 (8-inch) circles on one sheet of parchment and a third (8-inch) circle on the second sheet of parchment. Turn paper over (circles will be visible).
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3
Beat whites with cream of tartar and salt in a large bowl with an electric mixer at medium speed until they hold soft peaks. Beat in sugar, 1 tablespoon at a time, then increase speed to high and continue beating until whites hold stiff, glossy peaks, about 4 minutes in a stand mixer or 8 to 10 minutes with a handheld. Divide meringue mixture among 3 circles, spread evenly within lines, and gently smooth tops. Bake until firm and very pale golden, about 2 hours, then let stand in turned-off oven (door closed) until firm, about 1 hour. While meringues bake, put ice cream in refrigerator to soften.
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4
Slide parchment sheets with meringue layers onto racks to cool completely. Carefully lift meringue and peel off parchment.
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5
Whisk together lemon juice, zest, sugar, yolks, whole eggs, and salt in a 2-quart heavy saucepan. Stir in butter and cook over moderately low heat, whisking frequently, until mixture is thickened and just reaches a boil, about 6 minutes. Transfer lemon curd to a bowl and chill, its surface covered with plastic wrap, until cold, at least 1 hour.
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6
Line springform pan with 2 crisscrossed sheets of plastic wrap. Arrange 1 meringue layer (trimming if necessary) flat side down in springform pan and spread 1 cup ice cream over it, then evenly spread half of lemon curd over ice cream. Cover curd with another meringue layer, then spread remaining cup ice cream over meringue. Spread remaining curd on top, then cover curd with remaining meringue layer, flat side down. Cover with plastic wrap and freeze until firm, at least 4 hours.
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7
About 1 hour before serving, put cake in refrigerator to soften slightly. Remove side of pan and transfer cake to platter, then cut into wedges with a long serrated knife.
