This delicate cake layer was undoubtedly named after Margherita di Savoia, Italy’s second and most popular queen. As in a French génoise, the eggs and sugar are warmed before being whipped, so that they can absorb the air that leavens the batter while the cake is baking. These layers keep extremely well. Double-wrap them in plastic wrap and store in the refrigerator up to 5 days or in the freezer up to 1 month.
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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