Preheat the oven to 375°. Beat the egg whites with a hand-held mixer until they develop soft peaks. Then add ¼ cup sugar and beat until stiff peaks develop, another 4-5 minutes.
In a separate bowl, beat egg yolks and the remaining ¼ cup of sugar until they are pale and fluffy. Fold the whites into the yolks and mix delicately. Add vanilla, cornstarch, flour, and salt. Fold until just combined, do not over-mix.
Line a baking tray with parchment paper. Pour batter into a piping bag and pipe 4-inch cookies on the baking sheet. Bake for 10-12 minutes. Remove from oven and allow to cool completely.
Notes
When piping the ladyfingers, make them no bigger than an inch and a half. They will expand slightly.
Watch the edges of the ladyfingers when they're baking. You want the edges to be just golden brown.
While mixing the egg yolks and whites, be very delicate. Folding it in carefully is what keeps them fluffy. The same goes for when you're adding flour, cornstarch, salt, and vanilla.