In a medium mixing bowl, whisk the yolks with the vanilla extract. Whisk in half the sugar and continue to beat until very light and frothy, about 5 minutes. 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. Beating faster, add the remaining sugar in a very slow stream, beating until the egg whites hold a firm peak. Fold the yolks into the whites. Sift the flour and cornstarch over the eggs in 3 additions, folding them in gradually. Do not over-mix the batter.
Pour the batter into a buttered and paper-lined 9 or 10-inch round pan, 2-inches deep and smooth the top. Bake the Pan di Spagna at 350 degrees for 30 to 40 minutes, until it is well risen and feels firm when pressed gently with the palm of the hand.
Immediately loosen the layer from the side of the pan with a small knife or spatula. Invert the layer and leave the paper stuck to it. Turn the layer right side up and cool it on a rack. The cake can be stored double-wrapped in plastic in the refrigerator for up to 5 days, or in the freezer.