Wash prunes and soak overnight in water.
Cook in the same water until prunes are semisoft but do not overcook.
Drain thoroughly over a bowl. (You can refrigerate and use this liquid to sweeten other dishes or chill and serve it as a beverage.) Remove stones from prunes.
Place a half walnut in the cavity of each prune.
Set on a rack and allow to dry slightly.
Store in the refrigerator.