Combine juice, zest, salt, cornstarch, and sugar in a small saucepan. Whisk till well incorporated.

Turn heat to medium high.

In a separate bowl, whisk egg yolks until frothy and paler.

When lime juice mixture starts to boil, temper the egg yolks by slowly stirring in half of the hot liquid. Then add the tempered eggs back to the saucepan. Turn heat down to low.

Let simmer 3 minutes while stirring.

Take off heat and add in butter. Stir till combined.

Pour through a fine mesh strainer to get smooth curd.

Store in fridge after cools.