Prep the chicken. Butterfly or pound the chicken breasts to an even 1/2-inch thickness. Pat dry and season both sides with salt and pepper.
Preheat the griddle. Heat the Blackstone to medium (about 375°F) for 5 minutes, then add a thin layer of oil.
Crisp the bacon. Cook the bacon until crispy, 2–3 minutes per side, then move to the cooler edge to keep warm.
Cook the chicken. Griddle the chicken 4–5 minutes per side, flipping once, until golden.
Glaze. In the last 2 minutes, brush both sides with BBQ sauce and cook until caramelized and the internal temperature reaches 165°F.
Toast and build. Toast the buns on the griddle. Spread mayo on the bottom bun, then layer lettuce, tomato, chicken, bacon, and onion. Add extra BBQ sauce, top with the bun, and serve.