In 2-quart saucepan, cook unsweetened chocolate, butter and hot water over medium heat, stirring occasionally, until melted and smooth. Remove from heat; cool 5 minutes. In large bowl, mix flour, brown sugar, baking soda and salt. Add melted chocolate mixture, beating just until blended. Add eggs, one at a time, beating until blended. Beat in sour cream and vanilla. Pour batter into pans.