Finely grate the zest of the lemons.
Squeeze the juice, discarding seeds.
In a double boiler over simmering but not boiling water, melt the butter.
Add the sugar, egg yolks, lemon zest, and lemon juice.
Stir constantly until the mixture is the consistency of thick custard, about 10 minutes.
If it is not to be used right away, pour the lemon curd into sterilized jars and store in the refrigerator.
Extra lemon curd can be used to fill cookies or cakes, or folded into whipped cream for a light dessert.