01 - Combine the shredded chicken breast with BBQ sauce in a medium bowl, mixing until the chicken is evenly coated throughout.
02 - Blend the shredded cheddar and mozzarella cheeses together in a separate bowl for even distribution.
03 - Lay out all bread slices and spread softened butter generously on one side of each slice.
04 - Place 4 bread slices butter-side down. Layer each with BBQ chicken mixture, cheese blend, and optionally add red onion and cilantro. Top with remaining bread slices, butter-side up.
05 - Heat a large skillet or griddle over medium heat until properly preheated.
06 - Place sandwiches in the preheated skillet, cooking in batches if necessary. Grill for 3-4 minutes per side, pressing gently with a spatula, until bread turns golden brown and crispy with cheese completely melted.
07 - Transfer sandwiches from skillet, allow to cool briefly, then slice diagonally and serve while warm.