Arrange cucumber slices o plate. Sprinkle vinegar and salt over. Let mixture stand for 30 minutes. Drain cucumbers and pat dry. Light butter one side of each bread slice. Overlap cucumbers in 2 layers atop 12 bread slices. Sprinkle green onion over. season with pepper. Top each with slice of buttered bread, buttered side down. Cut sandwiches into rounds using 1 1/2 inch round cookie cutter