In a medium saucepan over medium-high heat, combine sugar, juice and salt; cook just until sugar dissolves, adjusting the heat down if needed to ensure mixture does not boil. Put egg yolks in a heat-safe glass bowl; stream in a few tablespoons of hot syrup very, very slowly, whisking constantly to temper the eggs. Add remaining syrup to the egg mixture very slowly, a few tablespoons at a time, whisking constantly. Pour the curd back into the saucepan and cook, stirring constantly with a silicone spatula, over medium-low heat until the mixture reaches 170° and the spatula leaves a faint, disappearing trail when dragged across the bottom (11-15 min.). Remove cooked curd from the heat and stir in cold butter a bit at a time until melted. Strain; move to a storage container. Press a bit of plastic wrap onto the surface of the curd. Cool completely in the refrigerator. May be stored up to 10 days.