Combine the sugar and lime zest in a medium bowl. Mix well to combine and set aside for 5 minutes. Measure out the lime juice and cut the butter into pieces.

In the top of the double boiler whisk together the egg yolks and whole eggs. Slowly whisk in the sugar and zest, blending until smooth. Stir in the lime juice. Add the butter pieces in one at a time, stirring after each addition.

Continue stirring gently but continuously with a whisk or spatula to prevent the mixture from sticking to the bottom. Cook until the lime curd reaches 170 degrees. It took mine about 10 minutes..

Once the curd has reached 170 degrees remove from heat and place on the counter. Continue stirring for 5 minutes to thicken the curd.

Spoon into a container with a lid and refrigerate until ready to use. Can be refrigerated for up to 3 weeks.