Zest the lemons and mince the zest. Set aside. Juice the lemons. Set aside.
Combine the zest, juice, butter and sugar. Place in a small saucepan or double boiler.
Cook, stirring occasionally until the sugar is dissolved.
Beat the eggs with a whisk. Add a small amount (2 tablespoons) of hot juice mixture to eggs to temper the eggs.
Now add the egg mixture very (read very) slowly to the juice mixture. Make sure the heat is VERY low.
Cook, stirring constantly until mixture thickens. DO NOT BOIL.
Cool slightly and pour into jars, custard cups or a sealable plastic storage bowl.