01 - Toss shredded chicken with BBQ sauce in a medium bowl until thoroughly coated. Set aside while preparing remaining components.
02 - Combine cheddar and mozzarella cheeses in a separate bowl, mixing evenly for consistent melting.
03 - Spread softened butter on one side of each bread slice. Ensure complete coverage for even golden browning.
04 - Place four bread slices butter-side down. Layer each with BBQ chicken mixture, cheese blend, and optional onion or cilantro. Top with remaining bread slices, butter-side facing up.
05 - Heat large skillet or griddle to medium temperature, allowing surface to become evenly hot for consistent cooking.
06 - Transfer sandwiches to heated surface. Cook 3–4 minutes per side, pressing gently with spatula, until bread turns deep golden brown and cheese melts completely. Work in batches if necessary.
07 - Remove from heat and let rest 1–2 minutes before slicing. Serve while warm and cheese remains gooey.