01 - Combine shredded chicken with BBQ sauce in a medium bowl, mixing until evenly coated. Set aside until ready to assemble.
02 - Mix cheddar and mozzarella cheeses in a separate bowl for even distribution across sandwiches.
03 - Lay out all bread slices and spread softened butter evenly over one side of each slice.
04 - Place four bread slices butter-side down. Layer each with one-quarter of BBQ chicken, followed by generous cheese mixture. Add sliced onion and cilantro if using. Top with remaining bread slices, butter-side up.
05 - Heat large skillet or griddle over medium heat until hot.
06 - Place sandwiches in hot skillet, cooking in batches if necessary. Grill 3–4 minutes per side, pressing gently with spatula, until bread turns golden brown and crispy with cheese completely melted.
07 - Transfer sandwiches to cutting board, let cool briefly, then slice diagonally and serve while warm.