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1
In a small bowl, whisk the flour and cornstarch.
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2
In a large heatproof bowl, whisk the egg yolks and whole egg with 3/4 cup of the milk and three-fourths of the flour-cornstarch mixture until smooth.
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3
Gradually whisk in the remaining flour mixture until smooth.
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4
In a medium saucepan, combine the remaining 1 1/4 cups of milk with 1 1/2 teaspoons of the granulated sugar and bring to a boil, whisking.
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5
Slowly pour the hot milk into the egg mixture and whisk constantly until smooth.
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6
Whisk in the butter until melted.
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7
Add the lemon juice and zest and fold in the challah.
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8
Refrigerate until cooled.
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9
Preheat the oven to 375 and butter a 9-by-13-inch baking dish.
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10
Dust with granulated sugar, tapping out the excess.
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11
In the bowl of a standing mixer fitted with a whisk, beat the egg whites with the cream of tartar until soft peaks form.
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12
Gradually add the remaining 3/4 cup of granulated sugar in a steady stream and beat at high speed until the whites are firm and glossy.
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13
Fold the meringue into the challah custard until no streaks of white remain.
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14
Transfer the mixture to the prepared baking dish.
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15
Bake the souffle in the center of the oven until risen and golden, about 45 minutes.
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16
Let rest for 10 to 30 minutes.
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17
Dust with confectioners' sugar and serve.
