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1
Bring milk just to a simmer in a 1 1/2- to 2-quart heavy saucepan, then remove from heat.
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2
Beat together yolks, sugar, and vanilla in a medium bowl with an electric mixer at high speed until thick and pale, 3 to 6 minutes. Reduce speed to low and add cornstarch, mixing until incorporated. Add hot milk in a slow stream, mixing until smooth.
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3
Transfer custard to same saucepan and bring to a boil, whisking constantly, then reduce heat and simmer, stirring constantly, 2 minutes. (Mixture will be thick.) Remove from heat and transfer to a large bowl, then whisk in zest and juice. Cover surface with round of parchment or wax paper and cool to room temperature, about 1 hour.
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4
Put oven rack in lower third of oven and preheat oven to 375u00b0F.
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5
Butter souffl dish, then sprinkle with sugar, turning to coat all sides and knocking out excess.
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6
Beat egg whites with salt in another large bowl using cleaned beaters at medium-high speed until foamy. Add cream of tartar and increase speed to high, then beat until egg whites just hold stiff peaks.
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7
Stir one fourth of whites into lemon custard to lighten, then fold in remaining whites gently but thoroughly. Pour mixture into prepared souffle dish, then wrap paper strip tightly around outside of souffle dish, forming a collar that extends at least 4 inches above rim. Tape overlapping ends together.
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8
Bake souffle until puffed and golden in spots, 35 to 45 minutes. Remove collar and serve souffle mmediately.
