Move the oven rack to the middle position and preheat to 350°F (180°C).
Sift cake flour and set it aside.
In a stand mixer bowl, combine egg whites, sugar, and vanilla extract. Use the whisk attachment. Start mixing on low for 1 minute, then increase to medium-low (speed 4 on a KitchenAid) and whip for 3 minutes until the mixture is dense and dark.
With the mixer running, add lemon juice and salt. Increase speed to medium (speed 6 on a KitchenAid) and whip for another 3 minutes until the mixture is thin and foamy.
Increase speed to medium-high (speed 8 on a KitchenAid) and continue whipping until the meringue is glossy and thick, about 2 to 4 minutes. The meringue should form soft peaks that hold their shape but are still flexible.
Sprinkle the sifted cake flour on top of the meringue. Gently stir with a spatula to roughly combine, then fold the mixture by scraping from the bottom up until no flour pockets remain.
Scrape the batter into a 10-inch ungreased aluminum tube pan with a removable bottom. If you see any unincorporated flour, mix it in gently.
Bake for about 35 minutes, or until the cake is puffed, golden blond, and firm to the touch, or until it reaches an internal temperature of 206°F (97°C).
Invert the pan onto its stilts or cooling rack and let it cool upside down for at least 2 hours until completely cool.
Slide an offset spatula around the sides to loosen the cake. Remove the insert, then slide the spatula under the bottom to release the cake. Flip it onto a serving plate, gently pulling on the sides to release it from the center tube.