Add the sugar and lime zest to a small bowl. Rub them together with your fingertips for about 30 seconds, until fragrant.
Transfer the lime sugar to a small saucepan.
Add the lime juice, egg yolks, butter, and salt. Whisk to combine.
Place the saucepan over low to medium-low heat.
Cook for about 7–10 minutes, whisking or stirring constantly and scraping the bottom and sides of the saucepan.
Continue cooking until the curd is smooth, glossy, and thick enough to coat the back of a spoon.
Remove the saucepan from the heat immediately.
Pour the warm lime curd through a fine-mesh sieve into a clean bowl.
Let the curd cool for about 15 minutes.
Cover and refrigerate for approximately 2 hours, or until fully chilled and thickened.
Stir gently before serving.