Heat immersion circulator to 168°F. Have a clean mason jar (12 oz.) with a two-piece screw top lid ready.

In a food processor, blend sugar and zest for 1 minute or until sugar is of a fine texture.

With processor running, slowly add lemon juice through the feed tube followed by the egg yolks, one at a time, and then the melted butter. Blend well. The mixture should be smooth and creamy.

Pour mixture into mason jar. Add lid and close until just two-finger tip tight. Do not close lid tightly as air needs to escape once in the immersion circulator.

Submerge jar carefully into hot water and set timer for 1 hour.

After time is up, place the jar on the counter and cool for 30 minutes. Chill in the refrigerator. Use within two weeks.