From The Cake Bible by Rose Levy Berenbaum
2.5 to 3 lemons
1/2 cup + 2 TBSP sugar
pinch of salt
4 "large" egg yolks
4 TBSP (half a stick) butter
Soften the butter.
Shred or grate the zest of one of the lemons (makes about 2 tsp). Squeeze the lemons through a strainer into a liquid measuring cup; should make 3 fluid ounces (six TBSP) of juice.
Put the egg yolks and sugar into a heavy noncorrodible saucepan -- do not use plain aluminum! -- and beat together until well blended. Stir in the remaining ingredients except for the zest. Cook over medium-low heat, stirring constantly, until the mixture resembles a thin hollandaise sauce: it thickly coats the back of a wooden spoon but is still liquid enough to pour. It must not be allowed to boil or it will curdle. Whenever steam appears, remove briefly from the heat, stirring constantly, to keep it from boiling.
When the curd has thickened, pour at once through a strainer. Press with the back of a spoon until only coarse residue remains. Discard the residue. Stir the lemon zest into the strained curd and let cool, then seal airtight. The curd will continue to thicken as it cools.
If you want a perfectly smooth curd, you can add the zest at the beginning and let it be strained out.