In a medium saucepan, stir cornstarch, salt and sugar. Add water and stir again. Cook over medium heat, stirring constantly until mixture thickens, approximately 5 minutes. When mixture begins to boil, lower heat and continue to stir for 1 minute. Remove pan from the heat.

In a small bowl, place egg yolks and about 1/3 cup hot cornstarch mixture and stir together.

Whisk this mixture back into contents of the saucepan and return pan to medium heat. Stir constantly until mixture is very thick, about 5 minutes.

Remove from heat and stir in lemon juice and lemon zest.

Pour filling into a bowl or container and let cool for 30 minutes. Refrigerate for about an hour to firm. Remove from fridge and spread on cake layers.