Combine chocolate, butter and boiling water in a medium mixing bowl. Stir until chocolate is melted.
Combine flour, brown sugar, baking soda and salt in large mixer bowl. Gradually add chocolate mixture, beating until thoroughly blended. Blend in eggs, sour cream and vanilla. Beat one minute on medium speed.
Pour into greased and floured 9" x 13" pan. Bake at 350° for 35 to 40 minutes or until cake tester comes out clean. Cool completely.
Serves 12 to 15.