If canning, prepare a water bath canner, jars and lids before you begin making the lemon curd.

Wash the lemons and zest the outer skin to obtain ½ cup of zest.  

In a small bowl, combine the zest and sugar, mix well, and let set for 30 minutes.

Prepare a double boiler and begin to heat water in the bottom pan. Ensure the water in the bottom pan will not touch the top pan of the double boiler.

In the top pan of the double boiler, whisk together the egg yolks and whole eggs until thoroughly mixed.

Gradually add the zest and sugar mixture and continue to whisk, maintaining a smooth texture. Blend in the lemon juice and butter pieces.

Place the top pan with the lemon mixture over the bottom pan of the double boiler. Maintain a gentle boil in the bottom pan, allowing the steam to heat the mixture in the top pan.

Use a silicone spatula to gently scrape the bottom of the pan to prevent sticking as the mixture cooks. Stirring continuously, heat the lemon curd mixture until it reaches 170° F on a kitchen thermometer. 

Remove the top pan from the double boiler and place it on a heat-resistant surface on the counter. Stir the curd gently until it thickens, or about 5 minutes. 

Pour the curd through a mesh strainer into a glass or stainless-steel bowl and discard the zest collected in the strainer. 

Ladle or pour the curd into hot half-pint jars or containers, leaving 1/2 inch headspace. Seal with 2 part canning lids.

Load jars into a simmering water bath canner, and turn up the heat to bring the canner to a full rolling boil.

Process half-pint jars for 15 minutes if you are at an altitude of 0-1,000 feet. For 1,001 to 6,000 feet, process for 20 minutes; above 6,000 feet, process half-pint jars for 25 minutes. Do not can lemon curd in jars larger than half pint (8 oz).

After the canning time is complete, turn off the heat and allow the jars to sit in the canner for another 5 minutes before removing them to cool completely on a towel on the counter.

Store preserved jars of curd in a cool, dark place such as a pantry shelf and use within 3 to 4 months. Discard if browning or separation occurs.