1. Adjust oven rack to the lowest possible position. Preheat oven to 400°.
2. Grease 8 (6-ounce) ramekins, and dust lightly with 2 tablespoons sugar; refrigerate on a baking sheet until ready to use.
3. Scald milk and lemon zest over medium heat. Remove from heat, and cool.
4. With an electric mixer, beat remaining 1/2 cup sugar and 2 egg yolks in a large bowl 3 to 4 minutes or until light and fluffy, scraping down side of bowl several times. (Discard third yolk.) Gradually mix in flour until blended, scraping down side of bowl. Add milk mixture to egg mixture, and mix thoroughly. Add lemon juice and salt; place mixture back over low heat (or in a double boiler), and cook, stirring constantly, about 3 minutes or until thick and creamy. Remove from heat, and cool completely. (You can prepare the recipe to this point up to 2 days in advance, and store in refrigerator. Bring to room temperature before cooking.)
5. Beat whites with cream of tartar in a separate bowl at medium speed for about 10 seconds. Increase speed to medium-high, and beat 1 to 2 minutes or until soft peaks form. (Do not overbeat; if the whites appear dry and granular, they are overbeaten.)
6. Stir about one-quarter of egg whites into cooled egg mixture to lighten it. Fold in remaining whites gently, using a rubber spatula, just until incorporated. Do not overmix.
7. Pour mixture gently into prepared soufflé cups to top of rim. To help soufflés rise properly, run your finger around rim of dish to wipe edges. Bake at 400° for 10 minutes; reduce heat to 350°, and continue to bake 4 minutes or until exterior is set and center is slightly loose when shaken and soufflé has risen above dish. Dust with powdered sugar, and serve immediately.