HEAT oven to 350°F.
MICROWAVE chocolate and margarine in large microwaveable bowl on HIGH 2 min. or until margarine is melted. Stir until chocolate is completely melted. Add no calorie sweetener and granulated sugar; mix well. Beat in eggs, 1 at a time, until blended. Add vanilla; mix well. Add 1/2 cup flour and baking soda; mix well. Beat in remaining flour alternately with water.
SPOON into 24 paper-lined muffin cups.
BAKE 15 to 18 min. or until toothpick inserted in centers comes out clean. Cool 10 min.; remove from pans to wire racks. Cool completely. Serve topped with COOL WHIP.