In a medium bowl, whisk together sugar, eggs, egg yolks, and lime zest. Set aside.

In a medium saucepan, warm the lime juice until steaming. Do not boil.

Temper the hot juice into the egg yolk mixture, adding a small amount at a time and whisking well after each addition, until you’ve incorported all of the juice.

Return mixture to saucepan and cook over medium heat.

Bring to a low boil and continue cooking, stirring continually with a wire whisk. Cook until thickened, about 3-4 minutes.

Remove from heat. Whisk in the butter, and continue whisking until the butter melts into the curd.

Transfer curd to a clean bowl. Cover with clingfilm/plastic wrap, allowing the plastic to touch the top of the curd. (This prevents a tough “skin” from forming on top.)

Refrigerate until ready to use.

Before serving, use a wire whisk to ‘knock back’ the curd, whisking for 1-2 minutes or until smooth and creamy.

Store leftover curd in an airtight container in the refrigerator for up to three days.