In a small saucepan, combine cornstarch, sugar and salt. Gradually add water, stirring until smooth. Bring to a boil over medium heat, stirring constantly. Boil for 1 minute and then remove from the heat. Quickly stir half of hot mixture into egg yolks, mixing well. Return to saucepan, continuing to blend well and return to boiling. Boil for another minute. Remove from heat. Stir in yuzu or lemon juice, yuzu or lemon peel and butter. Pour immediately into pie shell.