01 - Toss shredded chicken with BBQ sauce in a medium bowl until evenly coated.
02 - Lay out bread slices and butter one side of each. Place buttered side down on preparation surface.
03 - Place 1 cheddar slice on 4 bread slices. Top with BBQ chicken mixture, onion slices, jalapeños if desired, mozzarella slice, and another cheddar slice. Complete with remaining bread slices, buttered side facing outward.
04 - Heat large skillet or griddle over medium heat. Cook sandwiches 3-4 minutes per side, pressing gently, until golden brown and cheese is thoroughly melted.
05 - Cut sandwiches diagonally in half and serve immediately while hot.