Combine butter, chives, mustard and cayenne in small bowl; blend well. Season to taste with salt and pepper. (Butter can be made 1 day ahead. Cover; chill. Bring to room temperature before continuing.) Spread chive butter on 1 side of each bread slice. Place chicken on buttered side of 2 bread slices. Top chicken with some watercress. Press remaining 2 bread slices, buttered side down, onto watercress. Cut each sandwich into 4 pieces. Arrange any remaining watercress on 2 plates. Top with sandwiches.