Heat half-and-half in your best saucepan. Heat it just to barely boiling, do not boil if you can help it. In a mixing bowl, whisk the egg yolks, sugar, cornstarch and salt. Whisk well until creamy, about two mins.

When the half and half is near boiling, remove from heat and SLOWLY whisk some (about a half cup or so) of half-and-half into egg yolk mixture, then add all the half-and-half whisk togehter. Pour back into saucepan and cook over medium heat, whisking continuously, until custard is thick, about maybe four mins - whatever you do, don't boil it. Remove the pan from heat and whisk in butter, lemon zest and lemon juice. Spoon custard into individual dishes. Cover with plastic wrap, directly onto the top of the custard (this prevents a gross skin from forming) and refrigerate for at least four hours.