In a large bowl, beat sugar and oil. Beat in egg yolks and vanilla. Combine dry ingredients; add to sugar mixture alternately with milk, beating well after each addition. In a large bowl, beat egg whites until stiff peaks form; fold into batter. , Pour into three greased and waxed paper-lined 9-in. round baking pans. Bake at 350° for 20-25 minutes or until a toothpick comes out clean. Cool for 10 minutes; remove to wire racks to cool., For filling, in a large saucepan, combine the sugar, cornstarch and salt. Stir in water until smooth. Cook and stir over medium-high heat until thickened and bubbly. Reduce heat; cook and stir 2 minutes longer. Remove from the heat. Stir a small amount of hot filling into egg; return to all the pan, stirring constantly. Bring to a gentle boil; cook and stir 2 minutes longer. Remove from the heat. Gently stir in lemon juice, zest and butter. Cool to room temperature without stirring. Cover and refrigerate. , In a large bowl, combine the butter, confectioners' sugar, lemon juice, zest and enough cream to achieve desired spreading consistency. Spread filling between cake layers. Frost top and sides of cake. Store in the refrigerator.