In a small saucepan, bring the water, chocolate and butter to a boil; boil for 1 minute. Remove from the heat; stir in sugar and vanilla until smooth. Set aside.
In a large bowl, cream shortening and sugar until light and fluffy. Beat in egg. Combine the flour, baking powder and salt; add to creamed mixture alternately with milk, beating well after each addition.
Pour into pastry shell. Carefully pour reserved chocolate mixture over filling. Sprinkle with nuts if desired. Cover edges of pastry with foil. Bake at 350° for 55-60 minutes or until a toothpick inserted in the center comes out clean.