The tartness of this sauce offsets the sweetness of our Mini Honey Fruitcakes.
Combine sugar, cornstarch, and 1/4 teaspoon salt in a medium saucepan. Whisk in water. Bring to a boil, whisking. Continue to boil, whisking, until clear and thick, about 6 minutes more.
Stir in butter, lemon zest, and citrus juices. Cook, stirring regularly, until smooth, 2 to 3 minutes more.
Sauce can be refrigerated for up to 2 weeks.