Combine sugar and cornstarch in a medium saucepan; stir in 1 cup boiling water. Cook over medium heat, stirring constantly, until sugar and cornstarch dissolve (about 3 minutes). Gradually stir about one-fourth of hot mixture into yolks; add to remaining hot mixture, stirring constantly with a wire whisk. Stir in lemon juice.
Cook, stirring constantly, until thickened. Remove from heat; stir in butter. Cool, stirring occasionally.