Orange Layer Cake

This is a fatless sponge — no butter, no oil, no milk — leavened almost entirely by the air beaten into the eggs. Everything depends on how gently the flour goes in.

• The hinge: foldingBeating the yolks and sugar to a thick ribbon, then folding whites and flour in by hand, is the whole leavening system. Once the flour is added, use a spatula and a light hand; stirring it smooth will deflate the batter and give you a rubbery, shallow cake.
• Sifted four timesNot a fussy flourish. Repeated sifting aerates the low-protein flour and distributes the soda and salt evenly, since there is no creamed butter to do that work. Sift at least twice, and sift it onto the batter rather than dumping it in.
• Pastry flourRoughly 8-9 percent protein. Substitute cake flour straight across, or make a stand-in by replacing 2 tablespoons of each cup of all-purpose flour with cornstarch.
• Why soda and not baking powderThe 4 tablespoons of orange juice plus lemon juice supply the acid the soda needs. Do not reduce the citrus or swap in a sweeter, less acidic juice without switching to baking powder, or the cake will taste faintly soapy.
• The tinAn "angel cake tin" is a tube pan. Grease it and line the base with parchment as directed, but note that this batter is only about 1 cup of flour — in a 9-inch tube pan it will bake very thin. A 6- or 7-inch deep round tin gives a layer tall enough to split.
• Oven and doneness375°F is right as written. Check at 25 minutes: the top should spring back and a skewer come out clean. Sponges of this size overbake quickly and turn dry and papery.
• SplittingCool completely before cutting the single cake horizontally, using a long serrated knife and a sawing motion. A warm sponge tears.
• YieldA modest cake, six to eight small servings. The orange cream filling and boiled orange frosting are referred to by name and so are separate recipes elsewhere in the book; both are called for here, and the cake is very lean if served plain.