Combine sugar, cornstarch, flour, and salt in a medium saucepan. Gradually add water, stirring well. Cook over low heat, stirring constantly, until thickened and bubbly.
Gradually stir about one-fourth of hot mixture into eggs; add egg mixture to remaining hot mixture, stirring constantly. Cook 2 minutes over low heat, stirring constantly.
Remove from heat. Add butter, lemon rind, and juice; stir until butter melts. Cool slightly; spoon into pastry shells. Cool completely. Garnish with a dollop of whipped cream and grated lemon rind, if desired.