Combine sugar, cornstarch and salt. Stir in boiling water. Cook over direct heat until mixture thickens and boils, stirring constantly.
Place in double boiler and cook 10 minutes. Beat egg yolks slightly; gradually beat in hot mixture until half of it has been added. Pour quickly into mixture in double boiler; cook 5 minutes, stirring constantly.
Remove from hot water and blend in lemon juice and peel and butter. Cool.