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1
Adjust an oven rack to the middle position and heat the oven to 375u00b0F (190u00b0C). Using an electric mixer, whip the egg whites and cream of tartar together on medium-low speed until foamy, about 1 minute. Slowly add 1/3 cup of the granulated sugar and the salt, then increase the mixer speed to medium-high, and continue to whip until stiff peaks form, 3 to 5 minutes. Gently transfer the whites to a clean bowl and set aside.
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2
Using an electric mixer (no need to wash the mixing bowl), whip the yolks and the remaining 1/3 cup sugar together on medium-high speed until pale and thick, about 1 minute. Whip in the lemon juice, zest, and flour until incorporated, about 30 seconds.
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3
Fold one-quarter of the whipped egg whites into the yolk mixture until almost no white streaks remain. Gently fold in the remaining egg whites until just incorporated.
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4
Melt the butter in a 10-inch ovenproof (not nonstick) skillet over medium-low heat. Swirl the pan to coat it evenly with the melted butter, then gently scrape the lemon souffle batter into the skillet and cook over medium-low heat until the edges begin to set and bubble slightly, about 2 minutes.
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5
Transfer the skillet to the oven and bake the souffle until puffed, the center jiggles slightly when shaken, and the surface is golden, 7 to 11 minutes. Using a potholder (the skillet handle will be hot), remove the skillet from the oven. Dust the souffle with the confectioners' sugar and serve immediately.
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6
Variation: Chocolate-Orange Souffle. Follow the recipe, substituting 1 tablespoon grated zest from 1 orange for the lemon zest, and 1/3 cup orange juice for the lemon juice. Gently fold 1 ounce finely grated bittersweet chocolate (about 1/2 cup) into the souffle batter after incorporating all of the whites in step 3.
