Pour sweetenedd condensed milk into top of double boiler. Add chocolate and salt.
Cook over rapidly boiling water, stirring constantly until mixture thickens and drops, rather than runs from spoon (about 4 to 8 minutes). (Cooking time varies with type of utensil used.)
Stir in water gradually, keeping mixture smooth.
Continue cooking, stirring occasionally, until mixture again thickens slightly, about 1 to 3 minutes. (If glassware double boiler is used, cover, do not stir, and cook for 3 to 5 minutes until mixture thickens slightly).
Remove from heat. Stir in vanilla. Pour into pastry shell.
Cool at room temperature for about 1/2 hour; then, chill in refrigerator for at least 3 hours.