In a medium heavy-bottom saucepan, whisk together the sugar, cornstarch, water, and salt. Set over medium heat and whisk until the mixture begins to boil (it will be quite thick), 4 to 7 minutes. Add the eggs yolks to the hot mixture and immediately whisk vigorously until thoroughly incorporated. Lastly, whisk in the lime juice, zest, and butter. Continue whisking until the mixture returns to a boil. Remove from the heat and pour into the crust. Let cool to room temperature, then refrigerate, covered with plastic wrap, until the filling is chilled and nicely set. (The tart can be made up to 24 hours ahead up to this point.)