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1
Place the lemon juice in a small nonreactive bowl; sprinkle the gelatin over. Set aside. Make an Ice water bath for next step.
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2
Heat the milk and 1/2 cup sugar in a medium saucepan over medium-low heat, stirring occasionally, until steaming and the sugar is dissolved, about 5 minutes. Meanwhile, whisk together the yolks, 2 tablespoons sugar, and cornstarch in a medium bowl until pale yellow and thickened. Whisking constantly, gradually add the hot milk to the yolks. Return the milk-egg mixture to the saucepan and cook, stirring constantly, over medium-low heat until the foam has dissipated to a thin layer and the mixture thickens to the consistency of heavy cream and registers 185 degrees on an instant-read thermometer, about 4 minutes. Pour the mixture through a mesh sieve and into a medium bowl; stir in the lemon juice mixture and zest. Set the bowl with the custard in a large bowl of ice water; stir occasionally to cool.
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3
While the custard mixture is chilling, in the bowl of a standing mixer fitted with the whisk attachment, beat the egg whites and cream of tartar on medium speed until foamy, about 1 minute. Increase the speed to medium-high; gradually add the remaining 2 tablespoons sugar and continue to beat until glossy and the whites hold soft peaks when the beater is lifted, about 2 minutes longer. Do not over beat. Remove the bowl containing the custard mixture from the ice water bath; gently whisk in about one third of the egg whites, then fold in the remaining whites with a large rubber spatula until almost no white streaks remain.
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4
In the same mixer bowl (washing is not necessary), using the whisk attachment, beat the cream on medium-high speed until soft peaks form when the beater is lifted, 2 to 3 minutes. Fold the cream into the custard and egg-white mixture until no white streaks remain.
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5
Pour into the prepared souffle dish (see note) or bowl. Chill until set but not stiff, about 1 1/2 hours; remove the foil collar, if using, and serve, garnishing if desired.
