Skillet Lemon Souffle...
Adjust oven rack to middle position and heat oven to 375 degrees. Using stand mixer, whip egg whites and cream of tartar together on medium-low speed until foamy, about 1 minute. Slowly add 1/3 cup sugar and salt, then increase speed to medium-high and continue to whip until stiff peaks form, 3 to 5 minutes. Gently transfer whites to clean bowl and set aside.
Using stand mixer (no need to wash mixing bowl), whip yolks and remaining 1/3 cup sugar together on medium-high speed until pale and thick, about 1 minute. Whip in lemon juice, zest, and flour until incorporated, about 30 seconds.
Whisk ¼ of whipped egg whites into yolk mixture until almost no white streaks remain. Gently fold in remaining egg whites until just incorporated.
Melt butter in 10-inch ovenproof skillet over medium-low heat. Swirl pan to coat evenly with melted butter, then gently scrape soufflé batter into skillet and cook until edges begin to set and bubble slightly, about 2 minutes.
Transfer skillet to oven and bake soufflé until puffed, center jiggles slightly when shaken, and surface is golden, 7 to 11 minutes. Using potholder (skillet handle will be hot), remove skillet from oven. Dust soufflé with confectioners’ sugar and serve immediately.