Combine 750ml water, 230g sugar,salt, lemon juice and lemon zest in a saucepan and bring to a boil.
Meanwhile, combine the remaining sugar,cornstarch, egg and remaining water until well incorporated.
When the lemon mixture begins to boil, remove the saucepan from the heat and add the egg mixture a tiny bit at a time while whisking furiously (to ensure the egg doesn't become scrambled).
Once all the egg is incorporated, return the saucepan to the heat and allow the mixture to boil until it becomes thick.
Once thick, immediately remove from the heat and stir in the butter.
Pour the filling over the cooled crust and allow the filling to cool and set.