Sift together flour, cream of tartar and baking powder four times. Beat
egg yolks with a rotary or electric beater until very thick, light and creamy.
Add the sugar, a few tablespoons at a time, and continue beating until mixture
is smooth and pale yellow. (This is a sponge cake, essentially, so thorough
beating is imperative.) Stir in lemon rind and lemon juice.
Beat egg whites and salt until egg whites stand in peaks. Fold into egg
yolk mixture alternately with the flour until well mixed.
Spoon into cake pans and bake in a preheated 325 degree F oven 20 to 25
minutes or until cake begins to pull away from sides of pans.
Loosen edges with a knife and turn out on cake racks to cool while you prepare
the Lemon Jelly Filling and Lemon-Orange Frosting.
Lemon Jelly Filling: Mix egg yolks with sugar, lemon rind and lemon juice
and cook over boiling water, stirring constantly, until sugar dissolves.
Add butter and continue cooking, stirring constantly, for 20 minutes or until filling
is smooth and very thick.
Cool, then spread between layers of cooled cake.
Lemon-Orange Frosting: Beat or work butter until it has the appearance of
thick cream. Stir in confectioners' sugar, a little at a time, and continue
working until mixture is very smooth.
Beat in egg yolk and lemon juice.
Stir in enough orange juice to make a spreadable frosting, then add grated lemon
and orange rind.