Microwave chocolate and butter in large microwaveable bowl on HIGH 2 min. or until butter is melted. Stir until chocolate is completely melted. Add sugar; beat with mixer until blended. Add eggs, 1 at a time, mixing well after each addition. Blend in vanilla. Combine 1/2 cup flour, baking soda and salt. Add to chocolate mixture; mix well. Add remaining flour alternately with buttermilk, mixing well after each addition.