In a small heavy saucepan, combine the sugar, cornstarch and flour. 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 mixture into egg yolks; return all to the pan, stirring constantly. Bring to a gentle boil; cook and stir 2 minutes longer. Remove from the heat. Gently stir in the juices, butter and zests. Transfer to a small bowl; cool for 10 minutes. Refrigerate until chilled.
For cake: In a large bowl, cream butter and sugar until light and fluffy. Add eggs, one at a time, beating well after each addition. Beat in vanilla. Combine the sour cream, juices and lemon zest. Combine the flour, baking powder, salt and baking soda; add to the creamed mixture alternately with sour cream mixture, beating well after each addition.
Pour into three greased and floured 9-in. round baking pans. Bake at 350° for 18-22 minutes or until a toothpick inserted in the center comes out clean. Cool for 10 minutes before removing from pans to wire racks to cool completely.
Place the cake circle on a baking sheet; layer with one cake layer and half of filling. Repeat layers. Top with remaining cake layer.
For meringue: In a large bowl, beat egg whites and cream of tartar on medium speed until soft peaks form. Gradually beat in sugar, 1 tablespoon at a time, on high until stiff glossy peaks form and sugar is dissolved.
Spread meringue over top and sides of cake. Bake at 350° for 28-32 minutes or until meringue is lightly browned. Carefully transfer to a cake plate. Refrigerate leftovers.
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