Combine sugar, cornstarch, and butter in a heavy saucepan; mix well. Stir in boiling water; cook over low heat, stirring constantly, until thickened.
Combine egg yolk and lemon juice; stir well. Gradually stir about one-fourth of hot mixture into yolk; add yolk mixture to remaining hot mixture, stirring constantly. Continue to cook until thickened. Chill glaze thoroughly before using.