In a small saucepan, mix sugar, cornstarch and salt; whisk in egg yolks and milk until smooth. Whisk in lemon juice until blended; bring to a boil over medium heat, stirring constantly. Cook and stir until thickened slightly, 2 minutes longer. Stir in lemon zest., Transfer mixture to a bowl. Cover and refrigerate until cold., To serve, in a small bowl, beat cream on high speed until soft peaks form. Fold into lemon mixture. Spoon into serving dishes. If desired, top with additional whipped cream and lemon slices.