Gently heat the zest and juice of the lemons with the sugar in a small saucepan, stirring constantly. Then let the butter stir into it.
Pour the mixture into a water bath bowl (over a saucepan of simmering, not boiling, water). Add the beaten eggs and mix over very low heat for about 25 minutes, until the lemon cream thickens enough to coat the back of a spoon. Do not allow the cream to boil or it will curdle.
Pour into sterilized jars and seal.
Let cool and then store in the refrigerator.