Add the egg yolks, zest and lime juice and whisk until well combined. Add small pieces of butter and cook over medium/low heat. (be very careful it doesn't get curdled).
Stir constantly for 6 minutes or until mixture is thick enough to hold a line on the back of a wooden spoon.
Transfer to a bowl, cover with plastic wrap and refrigerate. This freezes well. You can cut this in half if you need to.
Notes
Another thing you can do with this recipe is to give it as hostess gifts. Get some ball mason jars and some ribbon and make a homemade tag and give it as a hostess gift during the holidays or a birthday.