Wash prunes and soak for 1 hour in a small amount of water.
Simmer slowly in the same water until very soft.
All water should be evaporated.
Pit prunes and put through a sieve or food mill.
There should be about 1 cup puree.
Beat egg whites with salt until stiff.
Add sugar gradually and continue to beat until mixture stands in firm peaks.
Fold in the prunes lightly but thoroughly to mix.
Fold in rind and nuts and turn into a 5-cup casserole.
Place in a pan of hot water and bake in a slow oven (275° F.) for 1 hour.
Serve with custard sauce made by combining 2 well beaten yolks with 3 tablespoons of sugar, a pinch of salt, and 1 cup of scalded milk in the top of a double boiler and stir constantly over hot water until custard coats a metal spoon.
Stir in 1/2 teaspoon vanilla.