Instructions

for the sponge cake

Adjust oven rack to lower middle position and heat oven to 350 degrees. Grease two 8- or 9-inch cake pans and cover pan bottoms with a round of parchment paper. Whisk flours, baking powder, and salt in a medium bowl (or sift onto waxed paper). Heat milk and butter in a small saucepan over low heat until butter melts. Remove from heat and add vanilla; cover and keep warm.
Separate three of the eggs, placing whites in bowl of standing mixer fitted with whisk attachment (or large mixing bowl if using hand mixer or whisk) and reserving the 3 yolks plus remaining 2 whole eggs in another mixing bowl. Beat the 3 whites on high speed (or whisk) until whites are foamy. Gradually add 6 tablespoons of the sugar; continue to beat whites to soft, moist peaks. (Do not overbeat.) If using a standing mixer, transfer egg whites to a large bowl and add yolk/whole egg mixture to mixing bowl.
Beat yolk/whole egg mixture with remaining 6 tablespoons sugar. Beat on medium-high speed (setting 8 on a KitchenAid) until eggs are very thick and a pale yellow color, about 5 minutes (or 12 minutes by hand). Add beaten eggs to whites.
Sprinkle flour mixture over beaten eggs and whites; fold very gently 12 times with a large rubber spatula. Make a well in one side of batter and pour milk mixture into bowl. Continue folding until batter shows no trace of flour, and whites and whole eggs are evenly mixed, about 8 additional strokes.
Immediately pour batter into prepared baking pans; bake until cake tops are light brown and feel firm and spring back when touched, about 16 minutes for 9-inch cake pans and 20 minutes for 8-inch cake pans.
Immediately run a knife around pan perimeter to loosen cake. Cover pan with large plate. Using a towel, invert pan and remove pan from cake. Peel off parchment. Re-invert cake from plate onto rack, (see illustrations below). Repeat with remaining cake.

for the filling

Bring sugar, cornstarch, salt, and water to simmer in a large nonreactive saucepan over medium heat, whisking occasionally at beginning of process and more frequently as mixture begins to thicken. When mixture starts to simmer and turn translucent, whisk in egg yolks, two at a time. Whisk in zest, then lemon juice, and finally butter. Bring mixture to a steady simmer, whisking constantly. Remove from heat, and transfer to another container to cool to room temperature, placing a piece of plastic wrap directly on the surface of the filling to prevent a skin from forming. Let stand to room temperature. (Can be refrigerated overnight.) To ensure that lemon filling does not thin out, do not whisk or vigorously stir once it has set.

to assemble

Place one cake layer on a cardboard round on a sheet of waxed paper. Carefully spoon filling over layer and spread evenly up to cake edge. Place the second layer on top making sure layers line up properly. Sieve confectioners’ sugar over cake and serve.