Preheat the oven to 400°F (200°C). Lightly grease a large baking dish or oven-safe skillet.
Arrange the halved and seeded bell peppers in the prepared dish with the cut sides facing up.
Add the shredded chicken, mayonnaise, buffalo sauce, garlic powder, onion powder, salt, black pepper, nutritional yeast, and sliced green onions to a large mixing bowl. Stir until everything is evenly coated and creamy.
Taste the filling and adjust the buffalo sauce or salt as desired.
Divide the chicken mixture among the bell pepper halves, packing the filling gently into each pepper.
Cover the baking dish with foil and bake the Buffalo Chicken Stuffed Peppers for 30 minutes.
Remove the foil and continue baking for about 20 minutes, or until the peppers are tender and the filling is hot, bubbling, and lightly browned on top.
Drizzle with dairy-free ranch dressing and finish with sliced green onions and fresh herbs before serving.