This is a good filling to use between layer cakes, and is prepared using basically the same method as a lemon pie filling. There should be enough mixture to fill two layers. Prep time includes chilling time.
In a Pyrex or heatproof bowl whisk the eggs very well.
Using a whisk stir together sugar, cornstarch and a pinch of salt in a heavy 2-quart saucepan, the whisk in water, orange juice, lemon juice and zest until smooth; bring to a boil over medium heat, whisking constantly.
Reduce heat and cook at just a bare simmer whisking constantly for about 2 minutes (the mixture will be thick).
Add in HALF of the hot juice mixture into the eggs in a slow stream, whisking continuously.
Whisk the egg mixture back into the juice mixture in the saucepan (this is called tempering) cook over medium-low heat whisking just until it reaches a boil.
Remove pan from heat and add in butter, mixing until melted.
Cover the surface with buttered piece of wax paper (butter side down) and chill until cold about 1 hour.