Heat the oven to 375°F. Lightly grease 8 cups of a standard muffin tin.
Add the ground chicken, panko, egg, 2 tablespoons of the Buffalo sauce, celery, 2 tablespoons of green onion, garlic powder, and salt to a mixing bowl. Stir just until combined.
Divide the mixture among the 8 prepared muffin cups, filling each cup evenly. Bake for 20 to 25 minutes, until the muffins are cooked through.
While the muffins bake, place the cauliflower florets in a saucepan and cover them with water. Bring to a boil, then cook until the cauliflower is very tender, about 10 to 12 minutes. Drain thoroughly.
Mash the cauliflower with the butter, milk, and black pepper until smooth. For a creamier texture, blend it until no large pieces remain.
Brush the cooked muffins with the remaining 2 tablespoons of Buffalo sauce. Serve them with the cauliflower mash and garnish with the remaining green onion.