For cake: Combine water and chocolate in a small heavy saucepan; cook over low heat, stirring constantly, until chocolate melts. (Or melt in 30 second increments in microwave); Remove from heat; let cool.
Beat 2/3 cup shortening at medium speed until creamy. Gradually add 1 3/4 cup sugar, beating well. Add eggs, one at a time, beating well after each.
Combine 2 1/2 cups flour, soda and 1/2 teaspoon salt; add to shortening mixture alternately with buttermilk, beginning and ending with flour mixture. Mix after each addition. Add cooled chocolate and 1 teaspoon vanilla. Mix just until blended.
Pour batter into 3 greased and floured 9-inch cake pans. Bake at 350 for 20 to 25 minutes or until center is done upon testing. Cool in pans on wire racks for 10 minutes; remove from pans, and let cool completely on wire racks.
In a small saucepan, place 1/4 cup of the flour and gradually stir in milk. Cook over low heat, stirring constantly, until thick. Remove from heat; let cool completely.
Beat butter and 1/2 cup shortening at medium speed until creamy. Gradually add remaining 1 cup sugar, beating well. Add remaining 1 Tablespoon of Vanilla, remaining 1/4 teaspoon salt and flour mixture. Beat until smooth. Spread filling between cake layers. Spread Creamy Chocolate Frosting (directions below) on top and sides of cake. Garnish, if desired, with "curls" of chocolate.
For Creamy Chocolate Frosting: Combine in large saucepan, the last 3 cups sugar, 3/4 cup milk, 3/4 cup butter and the 3 oz. unsweetened chocolate. Cook over low heat, until chocolate melts. Then cook over medium heat, stirring constantly, until mixture comes to a boil. Boil for 1 minute, stirring constantly; REMOVE from heat and stir in 3/4 teaspoon vanilla. Beat at high speed in mixing bowl, WHICH IS SITTING ON TOP OF ABOUT 2" OF ICE WATER. I USE AN OBLONG METAL CAKE PAN. THIS IS IMPORTANT.) Continue beating until frosting is of good spreading consistency. Spread immediately on cake.