In a heavy saucepan, combine 5 tablespoons sugar, cornstarch, and lemon zest. Gradually stir in 1-1/3 cups water until blended. Bring to a boil; cook and stir for 1-2 minutes or until thickened. Remove from the heat; stir in lemon juice. Set aside.
In another heavy saucepan, combine the egg whites, cream of tartar and remaining sugar and water. With a portable mixer, beat on low speed over low heat for 1 minute. Continue beating until mixture reaches 160°. Pour into the bowl of a heavy-duty stand mixer. Beat on high speed until stiff peaks form. Fold into lemon syrup. Cover and refrigerate for at least 2 hours.
For sauce, in a small saucepan, combine sugar and cornstarch. Gradually stir in milk until smooth. Bring to a boil; cook and stir for 2 minutes until thickened. Remove from the heat. Stir a small amount of hot milk mixture into egg yolks; return all to pan, stirring constantly. Bring to a gentle boil; cook and stir for 2 minutes. Remove from the heat; stir in vanilla. Cover and refrigerate until chilled. Spoon lemon meringue into dessert cups; top with custard sauce.