In a saucepan, whisk together the cornstarch, sugar, and water until well combined.
Place the saucepan over medium heat and stir constantly until the mixture thickens, about 5-7 minutes.
In a separate bowl, whisk the egg yolks.
Gradually add the lemon juice and zest to the egg yolks, whisking continuously.
Slowly pour the lemon mixture into the saucepan with the sugar mixture, stirring constantly.
Continue to cook and stir until the mixture thickens and comes to a boil. Remove from heat and let it cool slightly.