Juice the lemons through a fine mesh strainer into measuring cup. Set Aside.
Separate Eggs, putting yolks into a small bowl.
Using the fine mesh strainer, strain the yolks through the strainer into a heavy bottomed saucepan.
Add the Sugar, Lemon Juice, Egg Yolks, and Egg. Whisk to combine.
Cook over low heat, stirring constantly with a wooden spoon. Cook until the mixture thickens, and coats the back of the spoon. You can test doneness by running your finger down the coated spoon, checking that the curd doesn't run into the space where your finger was.
Stir in butter pieces, a few pieces at a time. Once butter is incorporated, add the zest and stir to combine.
You can use immediately, or spoon into a jar/s and refrigerate for later use.
Enjoy by spreading on shortbread cookies, scones, pound cake, angel food cake, cheesecake, or poached fruits. Enjoy.