Preheat oven to 350°F. Microwave chocolate and butter in large microwaveable bowl on HIGH 2 min. or until butter is melted. Stir until chocolate is completely melted. Blend in sugar and vanilla. Add eggs, one at a time, beating with electric mixer on low speed after each addition until well blended. Add 1/4 cup of the flour and the baking soda; mix well. Add remaining 1 cup flour alternately with the water, beating until well blended after each addition.