Bring sugar, cornstarch, salt, and 2 1/2 cups water to a boil in a medium saucepan over high heat, whisking constantly. Cook, continuing to whisk, until mixture thickens, about 5 minutes.
Stir in butter and lemon zest and juice. Cook, whisking, until smooth, 2 to 3 minutes more. Remove from heat and transfer to a bowl to cool completely. Sauce can be refrigerated up to 2 weeks in an airtight container.