To Make Cake:  Beat egg yolks and water until thick and lemon-colored. Gradually fold in vanilla, flour, baking powder, and grated lemon peel. Beat egg whites until stiff, but not dry, and peaks form. Beat in egg yolk mixture. Spread batter into wax-paper-lined 11 x 15-inch jelly roll pan sprayed with non-stick coating.

Bake in preheated 400-degree oven 10 to 12 minutes. Remove from pan immediately and place on clean kitchen towel. Remove wax paper and sprinkle sweetener over cake. Cool cake, then unroll.

To Make Filling:  In small, heavy saucepan, combine salt, lemon juice, and egg yolks. Cook over low heat until thickened, stirring constantly. Remove from heat and stir in sweetener and lemon peel. Fold whipped topping into cooled filling mixture.

Spread unrolled cake with filling. Roll again. Refrigerate one to two hours.