Set a strainer over a medium bowl. In a small saucepan, combine half of the lemon zest with the lemon juice, sugar, egg, egg yolk, salt and 1 tablespoon of the butter. Cook over moderate heat, whisking constantly, until thickened, about 5 minutes. Immediately strain the curd into the bowl. Whisk in the remaining lemon zest and 2 tablespoons of butter. Press plastic wrap directly onto the surface and refrigerate until cool, about 15 minutes.