Beat the egg yolks until creamy and very pale. (You will no doubt think after the first few minutes of beating the yolks that they are light in color.
But beat just a bit longer it's amazing how pale they get before they're ready.
I beat mine at least 5 minutes at high speed with my electric mixer.) Slowly sift the sugar into the eggs and continue beating as the sugar is added.
Add the lemon juice and beat in, then put the mixture in a double boiler. (I have a very deep double boiler pot, so that my mixer blades fit into it, and I beat the mixture right in the pot in which I cook it.)
Set over boiling water and cook until the mixture holds its shape.
Stir constantly to keep from lumping or sticking.
Sixteen to 17 minutes should suffice.
Remove from boiling water and substitute cold water beneath the mixture.
Set aside.
Beat the egg whites until stiff but still moist (do not overbeat) and fold the custard mixture into the whites.
Then this whole creamy, frothy creation is poured into the baked pie shell and topped with the meringue described for Key Lime Pie.