Place lemon juice, eggs, sugar, salt and zest into the Vitamix container in the order listed and secure lid. Turn machine on and slowly increase speed to Variable 10, then to High. Blend for 5 minutes. Reduce speed to Variable 5 and remove the lid plug. Add butter, 1 piece at a time, through the lid plug opening incorporating butter completely between additions. Replace the lid plug and increase speed to Variable 10. Blend for 30 seconds. Chill before serving or allow to cool slightly and serve at room temperature.