Preheat the oven to 325 degrees F. Mix half the sugar with the flour in a spice grinder or mini food processor for 5 seconds. Remove to a small bowl and set aside.
Beat the egg whites with the vanilla, cream of tartar and salt in a large bowl, using a mixer on medium, until foamy. Add the remaining sugar (not with the flour) slowly. Beat just until soft, moist peaks form. Add the flour mixture, half at a time, folding in with a rubber spatula until incorporated. Pour the batter into the ungreased loaf pan. Bake 35 to 40 minutes.
Invert the loaf onto 2 cans so that the handles of the pan rest on the tops of the cans, allowing ventilation. After 45 minutes to 1 hour, remove the cake by sliding the knife around the edges. Cool completely and slice with a serrated knife.