Place a strainer over a medium-sized bowl near the stove.
Place the sugar in a small bowl and zest the lemons onto the sugar. Use your fingers to rub the sugar and zest together. Juice the lemons until you have 3/4 cup of juice.
In a nonreactive 3 quart saucepan, combine the whole eggs, eggs yolks, and sugar and whisk until well blended. Stir in the lemon juice and butter.
Cook the mixture over medium heat, whisking constantly until the mixture is thickened. As you cook the mixture the butter will melt. When the mixture has thickened and reached a temperature of 185 degrees F pour it into the strainer. Press the curd through the strainer with a spatula to remove the solids.
Cover the surface of the curd with plastic wrap, gently pressing the plastic wrap onto the surface of the curd. Place the curd in the refrigerator and chill.
The lemon curd is good in the refrigerator for 2 weeks and can be frozen for up to 1 year. If freezing the curd package it in a freezer container after it has chilled. When ready to use place it in the refrigerator to thaw for 24 hours before intended use.
Notes
Zesting the lemons. When citrus fruit is zested the oils are released and a lot of the flavor and scent are in the oils. Zesting the fruit directly into the sugar ensures that you are capturing as much flavor as possible.Curd didn't thicken. If the curd hasn't thickened after chilling it in the refrigerator, you may not have cooked it long enough and to the right temperature.Use a non-reactive saucepan. A non-reactive saucepan is a stainless steel pan or a cast-iron pan coated with enamel. These materials will not react with the acid in the lemons. Do not use an aluminum pan. Aluminum will react with the acid in the lemons and produce a metallic taste.Do not walk away. Once you've started cooking the curd do not walk away while you're cooking it or it will burn. If you have to walk away for some reason, remove the pan from the heat.Dairy allergy. If you're allergic to dairy, you can leave out the butter. There will be a difference in texture, but the curd will still be creamy and tangy