Juice the lemons and grate three of the rinds, being careful not to grate beyond the skin and into the pith (white part)

Melt the butter on top of a double boiler over low heat, add the lemon and rind, and the sugar and salt

Once it's dissolved, add in the eggs, and continue to cook in the double boiler over simmering water.

When it is as thick as molasses or thick syrup , remove from heat , stir in the vanilla and pour it into clean jars.

Store in the refrigerator.