In a saucepan, bring water, butter and chocolate to a boil. Boil for 1 minute. Remove from the heat; cool. In a mixing bowl, combine flour, sugar, baking soda and salt. Add eggs, sour cream and vanilla; mix. Fold in walnuts. Pour into a greased 15-inch x 10-inch x 1-inch baking pan. Bake at 350 degrees for 20-25 minutes or until brownies test done. Cool for 10 minutes. For icing, melt butter and chocolate. Place in a mixing bowl; mix in 1-1/2 cups confectioners' sugar. Add milk, vanilla and remaining sugar; beat until smooth. Spread over warm brownies.