In a small, heavy bottom pot over medium heat, whisk together the egg yolks and sugar.

Add the lemon juice and zest and switch to stirring with a wooden spoon, so as not to aerate the curd.

Stir continually for 10-15 minutes, adjusting the heat as you go to ensure that it does not boil.

Your curd is done when it has thickened and coats the back of the spoon.

When you determine that it's finished, drop in the butter and stir until melted.

Position a fine mesh sieve over a glass or stainless steel bowl and pour the curd through it, to remove any bits of cooked egg and the zest.

Pour the curd into two prepared half pint jars, leaving 1/2 inch of headspace. If you want to process them for shelf stability, process them in a boiling water canner for 20 minutes (start the time when the water returns to a boil).

According to So Easy to Preserve, it is best to process only in half-pint jars or smaller, as they allow better heat infiltration.

Eat on toast, stirred into plain yogurt or straight from the jar with a spoon.