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1
Bring cream, 1/2 cup sugar, and zest to a simmer in a heavy medium saucepan over medium heat, stirring until sugar is dissolved.
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2
Whisk together yolks and remaining cup sugar in a bowl, then slowly add cream mixture, whisking to combine. Return mixture to saucepan and cook over medium heat, stirring constantly with a wooden spatula, until custard is thickened slightly and registers 175u00b0F on an instant-read thermometer (do not let boil). Immediately strain thourough a fine-mesh sieve into a metal bowl and quick-chill by setting bowl in a larger bowl of ice and cold water and stirring occasionally until cold, about 20 minutes.
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3
Churn mixture in an ice cream maker until partially frozen, 15 to 20 minutes, then, still churning, add lemon juice in a slow stream and continue to churn until frozen, about 10 minutes.
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4
While ice cream is churning, line bottom of a 13- by 9-inch baking pan with parchment paper and chill in freezer.
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5
Spread ice cream evenly in pan, then freeze, uncovered, until firm, at least 2 hours.
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6
Preheat oven to 325u00b0F with racks in upper and lower thirds. Lightly butter 2 large baking sheets, then line with parchment. Butter and flour parchment, knocking off excess flour.
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7
Stir together egg whites, sugar, vanilla, and salt in a bowl, then stir in coconut.
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8
Spread level tablespoons of coconut mixture into 3-inch rounds, 1 inch apart, on parchment-lined sheets.
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9
Bake macaroons, switching position of sheets halfway thourough baking, until golden in spots and around edges, 20 to 30 minutes.
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10
Cool on sheets on racks until firm, about 10 minutes, then remove from parchment with a metal spatula and cool completely, upside down, directly on racks.
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11
Using 3-inch cutter, cut out 9 rounds from ice cream, and, lifting with metal spatula, sandwich ice cream between macaroons. Freeze until ice cream is firm again, at least 1 hour. (You may have to assemble sandwiches in stages if ice cream gets too soft.) Once firm, serve immediately or wrap individually in plastic wrap and freeze until ready to serve.
