Add the egg yolks and sugar to a medium saucepan. Whisk for 1 to 2 minutes until the mixture looks pale yellow and smooth.

Pour in the key lime juice. Add the lime zest and salt. Whisk again until fully combined and smooth.

Place the saucepan over low heat. Cook slowly while stirring constantly with a whisk or silicone spatula to prevent the eggs from scrambling.

Continue cooking the curd for about 8 to 10 minutes or until thick enough to coat the back of a spoon. Do not let the mixture boil.

Remove the saucepan from the heat. Add the butter cubes gradually while stirring continuously until fully melted and silky smooth.

Pour the curd through a fine mesh strainer into a clean bowl to remove any cooked egg bits and create a smoother texture.

Press plastic wrap directly onto the surface of the curd and refrigerate for at least 2 hours until chilled and fully set.