In the top of a double boiler, combine eggs, sugar, zest, juice and salt.Heat the curd in the double boiler, over medium low heat, whisking constantly, until thick.Remove from heat and add butter, cut in pieces, stirring until melted.Let cool til it can go in fridge to chill. Keeps for up to a week.
Notes
Two Persian limes produced a bit more than an half cup of juice. I used the zest from those two limes, which was a bit more than called for. No one complained.