Place the prepared Brussels sprouts in a mixing bowl, drizzle with olive oil, and toss to coat thoroughly. Sprinkle in the garlic powder, smoked paprika, salt, and pepper, mixing until the seasoning is evenly distributed. Transfer the seasoned Brussels sprouts into the air fryer basket, ensuring an even layer. Cook at 400°F (200°C) for 10 minutes, pausing halfway through to toss for even crisping. Once golden and crisp, move to a serving bowl and garnish with parmesan if desired. Serve immediately.