Tata over at Poor Impulse Control was asked if she had her father's recipe. This is a very classic, very common recipe. It's where most very good pastry chefs would begin.
It is one of the better reasons I can think of to make friends with someone who has a lemon tree. Tree ripened lemons are essential for the richest flavors. Commercially picked lemons will not have the needed sugars or the complexity something like this requires. If you can't find backyard or homegrown lemons, hit the farmer's market. Grocery store lemons will only suffice if there is no other alternative.
2 teaspoons finely grated lemon zest
4 large egg yolks
3/4 sugar (again, i use the vanilla sugar that is in the pantry)
6 tablespoons fresh lemon juice (strained of pulp)
4 tablespoons softened sweet butter (cut into 1/4" cubes before softening)
pinch kosher or sea salt
Put the lemon zest into a medium mixing bowl and suspend a fine mesh strainer over it.
Using a heavy non-reactive saucepan, beat the yolks and sugar until well blended and creamy. Stir in the lemon juice, butter and toss in the pinch of salt. Cook these all together over a medium heat, stirring constantly. Hint: Use a wooden or a slotted spoon, not a whisk, we're not trying to whip up volume but to keep the ingredients moving.
The mixture will thicken and begin to coat a wooden spoon. Now comes the trick. This mixture must Not Be Allowed To Boil. That will make the yolks curdle and that means you have to throw it all out and start over. Whenever you see steam starting to appear, take the pot off the stove and keep stirring away, remembering to scrape the sides. When the mixture has gotten silky thick and reads 196° pour it at once into the strainer. Press the mixture through with the back of the spoon until only a thick residue remains. You can throw the residue away or smear it over a toasted bagel like I do. No one will call you foolish if you do. Gently stir into the zest, and while still hot place in a sterilized small canning jar. Seal tightly.
You can also substitute limes for the lemon to make the base for a wonderful version of Key Lime Pie that doesn't involve sweetened condensed milk. Use equal parts for the zest and the juice. Reduce the sugar to 1/2 cup and when cooking reduce the ending temperature to 185°.
This makes about a cup of finished curd. It. Rocks. Out. Loud.