1. Preheat oven to 325 degrees, with rack in lower third but not on bottom shelf. Prepare a 10-inch angel-food-cake pan: Using the pan as your guide, cut a circle from a piece of parchment paper; use it to line bottom of pan. Using a sieve, sift flour and 1/2 cup sugar onto another piece of parchment.

2. Set sieve over a bowl, and return mixture to sieve; sift again and set aside.

3. Place egg whites in a large mixing bowl. Beat on medium speed until frothy, about 1 minute. Add the cream of tartar, lemon juice, vanilla, and salt; continue beating until soft peaks form, about 2 1/2 minutes. With mixer running, add remaining cup sugar a little at a time, beating no longer than 1 minute.

4. Raise speed to medium-high; continue beating until firm, but not stiff, peaks form (when beater is lifted, only the tip of the peak should fall over slightly). Gently transfer egg-white mixture to a large, wide bowl. Sprinkle a third of the reserved flour mixture over the whites. Using a whisk, gently combine in a folding motion, allowing batter to fall through the whisk as you fold. Sprinkle remaining flour mixture over whites in two more batches; fold until just combined. Be careful not to overmix because the egg whites will deflate.

5. Using a large rubber spatula, transfer batter to prepared pan. Run a knife gently through the center of the batter to remove any large air bubbles. Bake until a cake tester inserted in the middle comes out clean and cake is springy to the touch, 45 to 50 minutes.

6. Remove pan from oven and invert onto its legs, making sure cake clears the surface of the counter. (If using a tube pan, invert it and hang over the neck of a bottle.) Let cool completely, 1 1/2 to 2 hours. To remove cake, reinvert pan, top side up. Run a knife around the inner and outer edges of the cake before releasing the bottom. Cake will keep up to 2 days at room temperature in an airtight container or wrapped in plastic. To serve, slice with a serrated knife.