Prune Souffle

From 1922:

One egg (white), 4 tablespoons mashed stewed prunes, 1 tablespoon sugar, few grains salt, few grains cream of tartar, ¼ teaspoon vanilla.
Wash prunes and soak in cold water for three or four hours. Cook in the same water till soft. Remove stones and rub through a sieve. Eight or ten prunes should make about 4 tablespoons mashed prunes. Add sugar, salt, cream of tartar and vanilla to prunes.
Beat white of egg till stiff and dry and fold prune mixture into it. Turn into a slightly buttered custard cup or small mold and bake 25 minutes in a pan of hot water in a rather hot oven. Use the yolk of the egg to make a boiled custard sauce.