Combine sugar and cornstarch in a medium saucepan; whisk in 1 cup boiling water. Cook over medium heat, whisking constantly, until sugar and cornstarch dissolve (about 2 minutes). Gradually whisk about one-fourth of hot sugar mixture into egg yolks; add to remaining hot sugar mixture in pan, whisking constantly. Whisk in lemon rind and juice.
Cook, whisking constantly, until mixture is thickened (about 2 to 3 minutes). Remove from heat. Whisk in butter; let cool completely, stirring occasionally.