01 - Combine shredded chicken and BBQ sauce in a medium bowl, mixing until evenly coated. Set aside until ready to assemble.
02 - Blend cheddar and mozzarella cheeses in a separate bowl for even distribution throughout sandwiches.
03 - Lay out all bread slices and spread softened butter evenly over one side of each slice.
04 - Place 4 bread slices butter-side down. Layer each with one-quarter of BBQ chicken mixture, generous portion of cheese blend, and optional onion and cilantro. Top with remaining bread slices, butter-side up.
05 - Heat large skillet or griddle over medium heat until hot.
06 - Cook sandwiches 3-4 minutes per side, pressing gently with spatula, until bread turns golden brown and crispy and cheese melts completely. Work in batches if necessary.
07 - Remove from skillet, let cool briefly, then slice diagonally and serve warm.