Process cream cheese in a food processor until smooth, stopping to scrape down sides.
Add finely chopped cucumber and next 4 ingredients; process until well blended.
Cut 2 rounds out of each bread slice using a 2-inch biscuit cutter. Spread cheese mixture evenly on one side of half of bread rounds. Top with remaining bread rounds. Carefully spread the cut outer edges of sandwiches with butter. Dip edges in 1/2 cup minced watercress, coating evenly. Cover and chill 2 hours.