In a glass or metal bowl, whisk together egg, egg yolks, sugar, lemon juice, lemon zest & salt. Set over the saucepan of simmering water, ensuring that the bottom of the bowl is not touching the water. Whisk the mixture constantly until it reaches 170 F. Immediately remove the bowl from the heat, add butter & whisk until smooth. Place over the bowl of ice water. Cover with plastic wrap making sure it is touching the lemon curd surface so a ‘skin’ will not form. Let mixture stand at room temperature until cooled & thickened.