Instructions

Place lemon juice in small nonreactive bowl; sprinkle gelatin over. Set aside.
Heat milk and 1/2 cup of the sugar in medium saucepan over medium-low heat, stirring occasionally, until steaming and sugar is dissolved, about 5 minutes. Meanwhile, whisk together yolks, 2 tablespoons sugar, and cornstarch in medium bowl until pale yellow and thickened, 3 to 5 minutes. Whisking constantly, gradually add hot milk to yolks. Return milk and egg mixture to saucepan and cook, stirring constantly, over medium-low heat until foam has dissipated to a thin layer and mixture thickens to consistency of heavy cream and registers 185 degrees on instant-read thermometer, about 4 minutes. Add chocolate to custard, stir until melted and fully incorporated. Strain into medium bowl; stir in lemon juice mixture and zest. Set bowl of custard in large bowl of ice water; stir occasionally to cool.
While custard mixture is chilling, in bowl of standing mixer fitted with whisk attachment (or in large mixing bowl if using hand mixer), beat egg whites and cream of tartar on medium speed until foamy, about 1 minute. Increase speed to medium-high; gradually add remaining 2 tablespoons sugar and continue to beat until glossy and whites hold soft peaks when beater is lifted, about 2 minutes longer. Do not overbeat. Remove bowl containing custard mixture from ice water bath; gently whisk in about 1/3 of egg whites, then fold in remaining whites with large rubber spatula until almost no white streaks remain.
In same mixer bowl (washing not necessary), with mixer fitted with whisk attachment, beat cream on medium-high speed until soft peaks form when beater is lifted, 2 to 3 minutes. Fold cream into custard and egg-white mixture until no white streaks remain. Pour into prepared soufflé dish or bowl. Chill until set but not stiff, about 1 1/2 hours (can be refrigerated up to 6 hours, see note); remove foil collar, if using, and serve, garnishing if desired.