One 9 by 2-inch round layer pan, or a 9-inch springform pan, buttered and the bottom lined with a disk of parchment or wax paper
Set a rack at the middle level of the oven and preheat to 350 degrees.
Sift together the cake flour, cornstarch, and cocoa through a fine-meshed strainer over a piece of waxed paper to break up any lumps in the cocoa. Set sifted ingredients aside with the strainer.
Whisk together the eggs yolks, sugar, and salt in the bowl of an electric mixer. Place the bowl over a pan of simmering water and continue whisking gently until the mixture is lukewarm, about 100 degrees. This should take only a minute or two.
Whip with the electric mixer on high speed until the mixture is cooled and increased in volume, about 3 to 4 minutes.
Remove bowl from mixer and sift in dry ingredients, in three additions gently folding each third in with a rubber spatula. Pour the batter into the prepared pan and spread the top even with a spatula.
Bake the layer about 30 minutes until it is well risen and the center is firm to the touch.
If necessary, loosen the layer from the side of the pan with a small knife or spatula and invert the cake onto a rack. Place another rack on the cake and invert again. Remove the top rack, so that the layer cools right side up, still on the paper