Melt the 4 ounces of chocolate; set aside to cool.
Sift flour with baking soda and 1/8 teaspoon salt; set aside.
In a large mixing bowl, using a hand-held electric mixer, cream butter with brown sugar until light and fluffy. Add the eggs one at a time, beating well after each addition. Scrape sides of bowl frequently. Beat in melted chocolate. Add 1 teaspoon vanilla to the cold water. Add sifted dry ingredients to batter, alternating with vanilla-water, beginning and ending with dry ingredients. Beat just to blend and make a smooth batter.
Turn batter into a greased and floured 10-inch Bundt pan or tube pan with a removable bottom. Bake at 350° for 55 to 60 minutes, or until a wooden pick or cake tester inserted in center comes out with just a few crumbs clinging. Cool the cake in the pan on a wire rack for about 15 minutes. (Cake might fall slightly during cooling) Loosen edges; invert onto rack to cool completely.
Make icing. Melt 6 ounces of semisweet chocolate in top of double boiler over medium heat. When chocolate is melted and smooth, stir in 1/8 teaspoon salt and 1/2 teaspoon vanilla and the sour cream. Whisk until well blended and silky. Pour icing over the cooled cake, letting excess drip down the sides.