In a medium saucepan, whisk together the sugar, cornstarch, and salt. Add the water, lemon juice, and lemon zest and whisk again. Set over medium-high heat and cook, stirring frequently, until this mixture comes to a boil. Remove from the heat and stir in the butter until melted.
1 cup granulated sugar, 4 Tablespoons cornstarch, 1/4 teaspoon salt, 1 1/2 cups water, 1/2 cup fresh squeezed lemon juice, 1 Tablespoon lemon zest, 2 Tablespoons salted butter