How to Make It
In a small bowl, whisk the flour and cornstarch. 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. Gradually whisk in the remaining flour mixture until smooth.
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. Slowly pour the hot milk into the egg mixture and whisk constantly until smooth. Whisk in the butter until melted. Add the lemon juice and zest and fold in the challah. Refrigerate until cooled.
Preheat the oven to 375° and butter a 9-by-13-inch baking dish. Dust with granulated sugar, tapping out the excess. In the bowl of a standing mixer fitted with a whisk, beat the egg whites with the cream of tartar until soft peaks form. 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. Fold the meringue into the challah custard until no streaks of white remain. Transfer the mixture to the prepared baking dish.
Bake the soufflé in the center of the oven until risen and golden, about 45 minutes. Let rest for 10 to 30 minutes. Dust with confectioners' sugar and serve.