This is a pretty easy thing. What you want to guard against — just as with all custards — is overcooking it, which will cause the egg proteins to clump up and fall to the bottom. You want to bring the mixture to a target of 196 degrees F, a degree or two more is no biggie, just don’t get it to boiling temperature. But truth be told even that isn’t a disaster…there are more than a few pastry pros out there who let their curd boil (the “curds” can be strained out and hey — at least you know your eggs are cooked completely!). I don’t recommend it. Use a thermometer. The formula is:
4 egg yolks
5 ounces sugar
3.25 ounces (by weight) lemon juice
2 ounces softened butter
2 teaspoons lemon zest
Whisk the yolks and sugar together until combined in a heavy saucepan (NOT aluminum…ever seen green eggs? You will.). Add the remaining ingredients — save for the zest — to the pan. Over medium-low heat, and while whisking constantly, bring the mixture to 196 degrees, until it coats the back of a spoon. Promptly strain the mixture through a fine sieve. Stir in the zest and allow to cool.