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1
Whisk together 1/4 cup sugar, cornstarch, and a pinch of salt in a 2-quart heavy saucepan.
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2
Whisk in 1/4 cup milk until smooth, then stir in remaining milk and zest.
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3
Bring to a boil over moderate heat, whisking frequently.
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4
Have ready a bowl filled with ice and cold water.
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5
Whisk together yolks and remaining 1/2 cup sugar in a large bowl.
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6
Gradually whisk in hot milk mixture, then transfer custard to saucepan.
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7
Cook, stirring constantly, until mixture registers 170F on an instant-read thermometer, 3 to 4 minutes.
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8
(Do not boil.)
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9
Immediately pour custard through a fine sieve into a metal bowl set in ice water.
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10
Stir in vanilla.
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11
Cool, stirring frequently.
