In a medium-size saucepan, bring sugar, lemon juice, salt and 1/2 cup water to a boil. In a medium-size bowl, whisk together cornstarch, egg yolks and 1/3 cup water. Quickly whisk egg yolk mixture into saucepan. Cook until thickened and clear yellow, about 1 to 2 minutes. Transfer to a bowl, cover surface with plastic and refrigerate.
In a second medium-size saucepan, whisk together the sugar and cornstarch. Slowly whisk in cream, half-and-half, egg yolks, vanilla and salt. Cook over medium to medium-high heat, stirring constantly, until consistency of sour cream, about 10 minutes. Transfer to a clean bowl, cover surface with plastic and refrigerate.
Heat oven to 350°F. Coat three 12 x 7 x 1-inch foil baking pans with nonstick cooking spray. Beat egg whites in a large bowl until stiff peaks are formed. Set aside. In a second large bowl, beat egg yolks, sugar and vanilla until light colored, about 2 minutes. Stir in lemon juice and zest, then fold in flour and baking powder. Fold in egg whites. Divide batter among prepared pans, a scant 3 cups per pan. Spread batter so that it is level. Bake at 350°F for 12 minutes. Immediately invert layers onto racks; remove pans. Cool completely.
Whip heavy cream and confectioners sugar together to stiff peaks. Fold into cooled pastry cream. Place one cake layer on serving platter. Top with half of the lemon curd (about 1 cup), spreading almost to edges of cake. Spread on a generous cup of pastry cream. Repeat with another cake layer, the remaining lemon curd and another generous cup of pastry cream. Spread top of cake with remaining pastry cream. Run a cake comb through the top, or simply swirl decoratively. Refrigerate before serving to firm slightly.
