In a small saucepan, whisk together the sugar, cornstarch, and salt.
Slowly whisk in the lime juice and water until smooth.
Stir in the lime zest.
Cook over medium heat, whisking constantly, until the sauce comes to a gentle boil and thickens.
Remove from the heat and whisk in the butter until completely melted.
Cool to room temperature, then refrigerate until chilled.
Right before serving, top each slice with a spoonful of whipped cream, a little extra lime zest, and a candied lime slice.