Combine prunes and water to cover in a heavy saucepan. Bring to a boil. Reduce heat; cover and simmer 30 minutes. Drain and cool.
Place prunes in container of an electric blender, and process until smooth. Set aside.
Beat egg whites (at room temperature) and salt until foamy; gradually add sugar, beating until soft peaks form.
Stir vanilla into prune puree; gently fold beaten egg whites into prune mixture. Spoon mixture into a 1 1/2-quart casserole. Bake at 325° for 15 minutes or until puffed and set. Cool to room temperature, and chill. Spoon into individual serving dishes. Sprinkle with pecans, and top with whipped cream.