Step 1: Zest the lemons and mince the zest. Set aside. Juice the lemons. Set aside.
Step 2: Combine the zest, juice, butter and sugar. Place in a small saucepan or double boiler.
Step 3: Cook, stirring occasionally until the sugar is dissolved.
Step 4: Beat the eggs with a whisk. Add a small amount (2 tablespoons) of hot juice mixture to eggs to temper the eggs.
Step 5: Now add the egg mixture very (read very) slowly to the juice mixture. Make sure the heat is VERY low.
Step 6: Cook, stirring constantly until mixture thickens. DO NOT BOIL.
Step 7: Cool slightly and pour into jars, custard cups or a sealable plastic storage bowl.