Chocolate Ice Box Pie

1 (9-inch) pie shell, baked
1 (14 ounce) can sweetened condensed milk
2 (1 ounce) squares unsweetened chocolate
1/4 teaspoon salt
1/2 cup water
1/2 teaspoon vanilla extract
1 cup whipping cream, whipped

Prepare baked pie shell. Cook first 3 ingredients in top of double boiler over hot water until thick. Add water slowly and let thicken again. Stir in vanilla extract. Pour into pie shell; refrigerate. Serve cold with whipped cream on top.