Preheat oven to 400 degrees F. Line a baking sheet with aluminum foil and spray with non-stick cooking spray. Lay the bacon slices on the lined baking sheet and cook in preheated oven for 12-15 minutes. Take bacon out of the oven, then lay bacon slices on a plate lined with paper towels to soak up bacon grease.
Warm the hot sauce in the microwave for 30 seconds, then set aside. In a large mixing bowl, mix together hot sauce, cream cheese, and cheddar cheese until cheeses have melted. Fold in the chicken, celery, and green onions. Season with salt and fresh black pepper to taste.
Prepare the sandwiches. Top one slice of bread with a generous amount of buffalo chicken mixture, two slices of bacon, a slice of cheddar cheese, then cover with the other slice of bread. Repeat until all the buffalo chicken mixture is used up.
Cook the sandwiches. Heat a large non-stick skillet over medium heat. Melt a pat of butter on the skillet. Place a sandwich on the skillet and cook until golden brown, about 2-3 minutes. Add another pat of butter to the skillet, turn the sandwich over, and cook for another 2-3 minutes, or until golden brown and cheese has melted. Repeat with remaining sandwiches. Serve immediately.