In a large saucepan, combine sugar and cornstarch. Stir in orange juice 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 1/2 cup hot mixture into egg yolks; return all to the pan, stirring constantly. Bring to a gentle boil; cook and stir 2 minutes longer. Stir in butter; keep warm.
In a small bowl, beat the egg whites on medium speed until foamy. Gradually beat in sugar, 1 tablespoon at a time, on high just until stiff peaks form and sugar is dissolved. Pour hot filling into pastry shell. Spread meringue over filling, sealing edges to crust.
Bake at 375° for 15 minutes or until the meringue is golden brown. Cool on a wire rack for 1 hour.
Refrigerate for at least 3 hours before serving. Garnish with orange zest if desired. Refrigerate leftovers.