Heat grill pan or Panini press to medium-high. Butter one side of each slice of bread or the outsides of two panini roll pieces. If including, spread mustard on the unbuttered side of one slice of bread or roll. Place three strips of brie cheese on the un-buttered side of both pieces of bread or roll. Place thinly sliced apple on one piece of bread atop brie. Top with four strips of bacon. Put the two sides together and place into a panini press or in a grill pan and press down. Grill until lightly browned on both sides. Serve at once.