Preheat oven to 350F. Microwave chocolate and butter in large microwaveable bowl on HIGH 2 min. or until butter is melted. Stir until chocolate is completely melted. Stir in sugar and vanilla.
Add eggs, 1 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.
Spoon evenly into 12 paper-lined medium muffin cups.
Bake 19 to 21 min. or until toothpick inserted in centers comes out clean. Cool in pan 10 min.; remove to wire racks. Cool completely.