Cake: Preheat oven to 350 degrees. Grease and flour three 9-inch round cake pans. Melt butter in a medium saucepan. Add water. Bring to a boil over medium heat, stirring occasionally. Remove from heat. Add chocolate, stirring until it melts. Stir in buttermilk, eggs, and vanilla. Set aside. Combine 1/2 cup flour and pecans, stirring to coat. Set aside. Combine remaining 3 cups flour, sugar, and baking soda in a large bowl. Gradually stir in chocolate mixture. Fold in pecan mixture. (Batter will be thin.) Pour batter into prepared pans. Bake 20 to 25 minutes, or until cake tester inserted in center comes out clean. Cool in pans on wire racks for 10 minutes. Remove from pans. Let cool completely on wire racks.
White Chocolate Frosting: Melt chocolate in top of a double boiler over low heat, stirring constantly. Remove from heat. Set aside to cool, about 10 minutes. Beat cream cheese and butter. Gradually add chocolate, mixing constantly, until blended. Slowly mix in powdered sugar until smooth. Add vanilla. Mix until blended.
Spread frosting on tops of two layers then stack layers on a serving plate. Use wooden picks to stabilize the two layers. Add third layer. Frost top and sides of cake. Refrigerate until ready to serve.