01 - Combine shredded chicken and BBQ sauce in a medium bowl, mixing until evenly coated. Set aside.
02 - Blend cheddar and mozzarella cheeses in a separate bowl for even distribution.
03 - Spread softened butter on one side of each bread slice.
04 - Place 4 bread slices butter-side down. Layer with BBQ chicken mixture, cheese blend, and optional onion and cilantro. Top with remaining bread slices, butter-side up.
05 - Heat a large skillet or griddle over medium heat.
06 - Cook sandwiches 3-4 minutes per side, pressing gently, until bread turns golden brown and crispy with cheese fully melted. Work in batches if necessary.
07 - Remove from heat, let cool for 1-2 minutes, slice diagonally, and serve warm.