01 - In a medium bowl, mix the shredded chicken with the BBQ sauce until evenly coated. Set aside.
02 - In a separate bowl, combine the cheddar and mozzarella cheeses until well mixed.
03 - Lay out all bread slices. Spread one side of each slice with softened butter.
04 - Turn 4 slices butter-side down. Top each with 1/4 of the BBQ chicken mixture, a generous layer of mixed cheese, and optional onion and cilantro if desired. Place remaining bread slices on top, butter-side up.
05 - Preheat a large skillet or griddle over medium heat.
06 - Place sandwiches in the skillet, working in batches if needed. Cook 3-4 minutes per side, pressing gently, until bread is golden and crispy and cheese is fully melted.
07 - Remove from skillet, let cool slightly, then slice diagonally and serve warm.