01 - In a medium bowl, toss the shredded chicken with BBQ sauce until evenly coated.
02 - Lay out the bread slices. Butter one side of each slice. Place the buttered side down.
03 - Add 1 slice of cheddar to 4 bread slices. Top with BBQ chicken, a few onion slices, jalapeños (if using), a slice of mozzarella, and another slice of cheddar. Finish with the second bread slice, buttered side facing out.
04 - Heat a large skillet or griddle over medium heat. Cook sandwiches for 3-4 minutes per side, pressing gently, until golden brown and cheese is melted.
05 - Cut sandwiches in half and serve hot.