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 and mixture is well blended. Add sugar and vanilla; beat with electric mixer until well blended. Add eggs, one at a time, beating well after each addition. Add 1/4 cup of the flour and the baking soda; mix well. Add remaining 1 cup flour alternately with the water, beating well after each addition.

SPOON into 12 paper-lined 2-1/2-inch muffin cups.

BAKE 19 to 21 min. or until wooden toothpick inserted in centers comes out clean. Cool in pan 10 min.; remove to wire rack. Cool completely. Tint whipped topping with food coloring; spread over cupcakes. Decorate as desired. Store in refrigerator.