Preheat oven to 325F. Prepare 12-cup volume bundt pan by coating evenly with soft butter, and sifting cake flour to cover entire inner surface of pan. Tamp out excess flour.
Sift the flour, first amount white sugar, and salt three times.
Using a stand mixer fitted with the whisk attachment, on medium speed whip the egg whites with water, lemon juice, cream of tartar, and vanilla extract till frothy.
Slowly stream in white sugar, 1T at a time, until all is added, gradually increasing the mixer speed to high. Whip until mixture is glossy and stiff peak.
Take off the mixer and sift the flour mixture over, folding with a large spatula in three additions, being careful not to deflate or overmix the batter.
Pour into prepared bundt pan, and bake in the bottom third of the oven for 15-20 minutes without opening the door.
Rotate cake gently and bake until a skewer comes out clean, about 30-35min total. Watch the cake carefully near the end so as to not over-bake.