Wash the canning jars.
Sterilize the canning jars.
Prepare the lids.
Fill the water canner with water (make sure jars will be covered by a couple of inches of water).
Heat the water to 180°F – that’s the temperature you want it to be when you first add the filled jars.
Start the water simmering in the bottom of the double boiler.
Take the top pot of the double boiler off the heat and use it to whisk the whole eggs and egg yolks.
Then add the sugar and lemon zest, whisking until mixed.
Add the lemon juice and the butter.
Place the pot back onto the base of the double boiler and heat – WHISKING CONSTANTLY
Whisk until the mixture is 170°F degrees (or it coats the back of a spoon).
Remove from the heat and whisk for another 5-10 minutes (you will feel it thicken some more).
Do not let the mixture boil!
Pour into the jars, leaving 1/2″ headspace.
Center the canning lids and screw the lids shut.
Place in the water bath canner (which is at 180°F degrees) and then bring to a boil.
Boil for 15 minutes and turn off heat.
Leave jars in the canner as it cools, for 5 more minutes.
Allow the jars to cool and check the seals.