In the top of a double boiler, whisk together the lime juice and cornstarch. Add the sugar, zest, and egg yolks; whisk to combine. Bring the bottom of the double boiler to a simmer. Using a whisk, stir the mixture until it’s thick enough to coat the back of a wooden spoon, about 20 minutes.

Whisk in the butter, one or two pieces at a time. When all the butter is combined, transfer the curd to a clean bowl. Cover the surface of the curd with plastic wrap and place in the refrigerator to cool.