Butter and line with parchment paper a 9-or 10-inch round cake pan that is 2 inches deep.
In a medium mixing bowl, whisk the yolks with the vanilla. Whisk in half the sugar and continue to beat until very light and frothy, about 5 minutes, either by hand, with a hand mixer set at medium speed, or in a heavy-duty mixer fitted with the whip.
Combine the flour and cornstarch and sift once to aerate.
In a clean, dry bowl, beat the egg whites with the salt until they hold a very soft peak, either by hand, with a hand mixer set at medium speed, or in a heavy-duty mixer fitted with the whip. Beating faster, add the remaining sugar in a very slow stream, beating until the egg whites hold a firm peak.
Fold the yolk mixture into the whites with a rubber spatula. Sift the flour and cornstarch over the eggs in 3 additions, folding them in gradually. Do not overmix the batter.
Pour the batter into the prepared pan and smooth the top. Bake at 350 degrees for 30 to 40 minutes, until it is well risen and feels firm when pressed gently with the palm of your hand.
Immediately loosen the layer from the side of the pan with a small knife or spatula. Invert the layer onto a rack and leave the paper stuck to it. Turn the layer right side up and cool it on a rack.
Unless you are going to use the layer within a few hours, double-wrap tightly in plastic and keep in the refrigerator up to 5 days, or freeze.