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