Preheat oven to 325º F. Line cupcake pans with 14 paper liners.
In the bowl of a food processor fitted with the blade, whirl the sugar to make the grains finer (about 1 minute). Remove half of the sugar to a small bowl and set it aside.
Add cake flour to the food processor and whirl to combine the sugar and flour. Set aside.
Wipe the bowl and whisk of a standing electric mixer clean before adding the egg whites. Beat the egg whites at low speed until the eggs begin to froth.
Add the cream of tartar and salt to the egg whites and increase the speed to medium. Beat until the whites form soft, billowy, slightly translucent mounds. Keeping the mixer on medium-speed, slowly add the sugar, one tablespoon at a time.
Continue beating on medium speed until the whites are soft, glossy, and form very soft peaks that flop over on themselves. Take care not to overbeat; the batter should still flow when you tilt the bowl.
Add vanilla and lemon juice and beat until just blended.
Sift a fine layer of the flour-sugar mixture over the batter (no more than 2 tablespoons at a time) and fold it in gently using a large silicone spatula. Repeat with remaining flour-sugar mixture.
Using a large ice cream scoop or spoon, add batter to the prepared cupcake liners, filling them almost to the top.
Bake until the cupcakes are pale golden and a toothpick inserted in the center comes out clean, about 20 minutes. Let the cupcakes cool for about 10 minutes in the pan before removing them to a wire rack to cool completely.