Whisk egg yolks, zest, lemon juices, and sugar in a medium-size, heavy saucepan. Cook over medium heat, whisking constantly, until mixture is thick enough to coat back of spoon, 8 to 10 minutes.
Remove from heat. Add salt and butter, whisking until smooth. Pour through a fine sieve into a medium bowl; discard solids. Press plastic wrap directly onto surface of curd to prevent a skin from forming. Refrigerate until firm and cold, at least 1 hour and up to 1 day.