Add ½ cup sugar, ⅔ cup lime juice, and zest from 1 lime in the top of a double boiler (or use a metal bowl that sits on top of a pan filled with an inch of water). Stir over medium-high heat until the butter melts.
In a separate medium bowl, add 2 egg yolks and whisk together.
1 Tbsp at a time, add the heated lime juice mixture to the egg yolks whisking vigorously the whole time. Continue to add the heated lime juice in slowly and steadily until about half the mixture is added. Then you can add in larger quantities.
Once the mixtures are combined, add back to the double boiler and cook until the lime mixture thickens and coats the back of a spoon, 45-60 minutes.
Add remaining 2 Tbsp of butter to the lime curd and stir until melted and smooth.
Allow to cool slightly before transferring to a jar. Cover and refrigerate.