Preheat oven to 400°F. Line a large baking sheet with parchment paper and spray lightly with cooking spray.
In a large bowl, combine ground chicken, panko, egg, blue cheese, ranch seasoning, garlic powder, and ¼ cup of buffalo sauce. Mix gently with your hands until just combined—don't overmix or the meatballs will be tough.
Roll the mixture into 1½-inch meatballs (about the size of a golf ball) and place them on the prepared baking sheet, spacing them about 1 inch apart.
Bake for 20-22 minutes, until the meatballs are cooked through and golden brown. A meat thermometer should read 165°F.
While the meatballs bake, whisk together the remaining ½ cup buffalo sauce and melted butter in a large bowl.
Transfer the cooked meatballs to the buffalo sauce mixture and toss gently to coat evenly.
Serve warm with toothpicks, ranch or blue cheese dressing, and celery sticks on the side. These can be kept warm in a slow cooker on low for up to 2 hours!