Whisk together the egg yolks, salt and sugar, until well combined

In a medium saucepan, whisk together cornstarch, water, lemon juice, and zest, Cook over medium-high heat, stirring frequently, turn down the heat and let it simmer for 1 minute.

While stirring add egg mixture and give it a gentle boil for about 2 minutes. Remove from heat and stir in butter.

Let the filling cool a bit, pour into a clean jar and refrigerate until thickens, about 1 hour.