Combine the sugar, cornstarch, and salt in a small saucepan; whisk to blend. Add the boiling water and whisk to blend. Place the pan over medium heat and continue cooking, stirring constantly, until the mixture is thickened and clear.
Whisk the lemon zest and lemon juice into the sauce.
Remove the sauce from the heat and add the cubed cold butter a little at a time, while whisking constantly.
Serve the sauce warm, at room temperature, or chilled.