Remove several strips of peel (no pith) to be cooked with curd. Place double boiler on stove and bring water to a simmer. Place butter in pot along with peel. Cut lemons in half and squeeze out juice (should yield about 12 ounces/ add more lemons if needed).
Break eggs into strainer and push through with spatula. Combine strained lemon juice with eggs and add to pot.
Add sugar, stirring to combine all ingredients. Continue cooking and occasional stirring until mixture thickens. Pour mixture into a measuring cup with a spout so that you can fill jars easily.
Remove peel before filling jars. Place lids on jars and hand tighten bands. Let cool on counter for several hours and then store in refrigerator.