From 1922:
Five eggs (whites), 1 cup mashed stewed prunes, 1/3 cup sugar, 1/8 teaspoon salt, 1/8 teaspoon cream of tartar, 1 teaspoon vanilla.
It will take about one-third of a pound of uncooked prunes to make one cup mashed and stewed prunes. Wash prunes carefully through several waters. Cover with one cup of cold water, and let stand for an hour. Cook in same water till tender.
Remove stones and rub prunes through a colander. Add sugar and cook till the mixture begins to thicken. Remove from fire and let cool. Add salt, cream of tartar and vanilla to prunes. Add gradually to the whites of the eggs beaten till stiff and dry.
This means absolutely “stiff and dry”. The whites must stand up. They should be beaten on a large platter with a wire whisk. Put pudding mixture in a buttered pudding dish. Set in a pan of hot water and bake 25 minutes in a moderate oven.
Serve cold with boiled custard or whipped cream. One-half cup of English walnuts may be added the last thing before baking.