Combine the egg yolks, juice, ½ cup sugar, starch and salt in a medium pot. Whisk until well blended. Place pot over lowest heat setting, and cooking, whisking constantly, until the mixture is hot (but not boiling), about 3 minutes. Whisk in the butter, one piece at a time, letting each piece incorporate fully before adding the next one (the whole process should take about 5 minutes). Continue to cook over low heat, whisking constantly until the curd thickens up just enough to cling to the back of a spoon.

Remove from the heat, and spoon into a jar (pass through a fine mesh sieve, if desired). May be stored in the fridge for up to 1 month.