Combine sugar and cornstarch in a large, heavy saucepan; mix well. Add butter, lemon rind, juice, and egg yolks; stir well. Gradually add milk, stirring constantly. Cook over low heat, stirring constantly, until mixture is thickened. Remove from heat; chill 1 hour.
Fold in sour cream, and pour filling into pastry shells. Chill 4 hours. Garnish with whipped cream and chocolate curls, if desired.