1. Preheat the oven to 350ºF (180ºC). Have a 9-inch (23 cm) tube pan ready. (Do not use a non-stick tube pan.)
2. In a small bowl, sift together the flour, 1/2 cup sugar, and salt.
3. In the bowl of an electric mixer, begin to whip the egg whites on medium speed. When they become foamy, add the cream of tartar and lemon juice.
4. Increase the speed to high and continue to whip the egg whites until they just begin to hold their shape in soft, droopy peaks. Gradually whip the remaining 1 cup of sugar into the whites, 1/4 cup at a time. Do not overwhip; the egg whites should not be overly dry or stiff, but soft and cloud-like. At the last moment, mix in the vanilla.
5. With a rubber spatula, then fold the flour and sugar mixture into the whites gradually, a small amount at a time.
6. Spoon the batter in the tube pan, smooth the top, and bake for 45 minutes. Remove from the oven and immediately invert the tube pan over a cooling rack. If the pan doesn’t have “feet”, set it over the neck of a heavy bottle or overturned metal funnel, using the center hole of the pan to hold the cake, until cool.