1. Preheat the oven to 375°F (190°C) and adjust an oven rack to the middle position.
2. Using an electric mixer on medium-low speed, beat the egg whites and cream of tartar until foamy, about 1 minute. Slowly add 1/3 cup granulated sugar and the salt and then increase the mixer speed to medium-high and continue to beat until stiff peaks form, 3 to 5 minutes. Gently transfer the whites to a clean bowl.
3. Using the electric mixer (no need to wash the bowl), beat the yolks and the remaining 1/3 cup sugar on medium-high speed until pale and thick, about 1 minute. Beat in the lemon juice, zest, and flour until incorporated, about 30 seconds.
4. Fold 1/4 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.
5. Melt the butter in a 10-inch ovenproof (not nonstick) skillet over medium-low heat. Using a 10-inch skillet is essential to getting the right texture and height in the lemon souffle. 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.
6. 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. Don’t open the oven door during the first 7 minutes of baking this souffle, but do check the souffle regularly for doneness during the final few minutes of baking. Be ready to serve the souffle immediately after removing it from the oven. Using a potholder (the skillet handle will be hot), carefully remove the skillet from the oven. Dust the soufflé with the confectioners’ sugar and serve immediately.