In a small bowl, whisk together the egg yolks, lemon zest, lemon juice, sugar and salt. Transfer to a small saucepan.
Cook the mixture over low-medium heat, stirring constantly, until it is thick enough to coat the back of a spoon, 3 to 5 minutes.
Remove from heat and add the butter. Mix until smooth.
Transfer the curd to a small bowl and cover with plastic wrap.
Refrigerate until chilled, about 30 minutes.