Heat the oven to 375°F. Lightly grease 8 cups of a standard muffin pan.
In a large bowl, combine the ground chicken, egg, panko, 2 tablespoons of the Buffalo sauce, celery, 1 tablespoon of the green onion, garlic powder, and salt. Mix gently just until combined.
In a small bowl, mix the blue cheese with the remaining 2 tablespoons of Buffalo sauce. Press a little of this mixture onto 4 mozzarella cubes, then top each with another cube to make 4 stuffed centers.
Divide the chicken mixture into 8 portions. Press 4 portions into the bottoms of the prepared muffin cups, making a small well in each. Place one stuffed mozzarella center in each well. Cover with the remaining chicken mixture and pinch the edges together to seal the filling.
Bake until the muffins are cooked through and the centers are hot, about 25 to 30 minutes. Let them rest in the pan for 5 minutes before lifting them out.
Drizzle or serve the muffins with ranch dressing and sprinkle with the remaining green onion.