In a small bowl, combine butter, chives, and mustard. Spread bread slices with a layer of seasoned butter. Top 8 bread slices with 2 slices each prosciutto and a layer of sliced cucumber. Season with pepper. Sandwich with remaining bread slices, buttered side down. With a sharp knife, cut off crusts, then cut each sandwich in half on the diagonal. Serve immediately or transfer to a platter and refrigerate, covered with plastic wrap, up to 3 hours.