Preheat oven to 350 degrees and position rack in center of oven
Grease inside of cake pans with butter and flour/cakespray and for added measure, line bottom of pans with parchment paper
Combine flour with salt in a small bowl and set aside.
In a standing mixer, whip the egg whites on medium-low speed until foamy (not stiff) and then add the cream of tartar and continue mixing until the whites become opaque
Add ½ cup sugar slowly to the side of the bowl and increase the speed of the mixer until soft but not stiff peaks form. Scrape the contents to another bowl and set aside
Using the same bowl, which is now empty, add the egg yolks and whip at high speed for 3 to 5 minutes until lightened in color and the mixture "ribbons" when the beaters are raised
Slowly add the remaining sugar (ie, ½ cup), cold water, and vanilla into the egg yolks and mix until combined on low speed
Sift the flour onto the egg yolks in two additions, and stir gently with large spatula to combine.
Gently but thoroughly fold in the whipped egg whites
Transfer mixture to prepared pans and bake for about 20 to 30 minutes until cooked through; using 3 pans, I only needed 20 minutes.
Remove from oven, cool, and then run knife carefully around rim to loosen and invert to wire rack. After fully cooled, wrap and refrigerate or freeze.