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. Beat in eggs, sour cream, and vanilla; beat 1 minute on medium speed.
Pour into greased and floured 13x9-inch pan. Bake at 350F for 35 to 40 minutes or until cake tester comes out clean. Cool completely. Frost with chocolate buttercream frosting. Serves 10-12.
