Combine the eggs, egg yolks, salt, and sugar in a heatproof bowl or the bowl of an electric mixer. Whisk to mix. Place the bowl over a pan of gently simmering water and whisk until warm, about 100 to 105 degrees. Remove the bowl from the pan and whip on high speed with a hand mixer or a heavy-duty mixer fitted with the whip. Continue whipping until the egg mixture has lightened in color, cooled, and increased about 4 times over its original volume.
Sift the flour over the egg foam in 3 or 4 additions, folding it in gently but thoroughly with a rubber spatula. Pour the batter into the prepared pan.
Bake the layer at 325 degrees for about 30 minutes, until it is well risen, firm to the touch, and a deep golden color. Remove to a work surface and insert the point of a small knife between the layer and the side of the pan. Loosen the layer from the pan and invert it onto a rack. Leave the paper in place and immediately invert the layer again onto another rack, to cool. Cool to room temperature and double-wrap in plastic for storage.
For a chocolate layer, substitute
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