Get jars and lids ready for canning according to manufacturer's directions. This generally involves cleaning and heating the jars in warm water, and simmering the lids in a shallow pan for several minutes. Keep jars and lids warm until filling.
Whisk egg yolks and sugar in a small pot set over medium heat. Gradually add lemon juice, stirring with a wood spoon to avoid aerating the curd. Keep stirring for 10 to 15 minutes, adjusting heat to prevent boiling. Check for doneness with a metal spoon. When the curd is thick and coats the back of the spoon, it is ready. Stir in butter pieces.
Strain curd into a glass or stainless bowl through a mesh sieve to eliminate pieces of cooked egg. Whisk in zest.
Carefully pour the curd into prepared jars. Leave 1/2 inch between the curd and the top of each jar. Process the jars in a boiling water canner for 20 minutes (always start timing when the water begins to fully boil). Remove basket and allow jars to cool completely before handling.
A few notes: it is generally recommended to use only half-pint jars for this canning project, to ensure thorough heating. You can expect a shelf life of about two months. Refrigerate after opening. Once opened, the curd should keep for about a week. Check for mold or a metallic taste when evaluating freshness.