In a medium saucepan combine the sugar, cornstarch, and salt.

Whisk in the cold water until well-blended.

Whisk in the egg yolks, lemon peel, and lemon juice.

Place the pan on the stove over medium heat, whisking constantly until it begins to boil.

Whisk and let it boil for one minute, then remove from the heat and stir in the butter.

Pour the filling into a heat-proof dish and let it cool to room temperature without stirring.

Store the filling covered in the refrigerator to chill before using.