Preheat the air fryer to 400°F (200°C).: Your nose will pick up a faint metallic warmth from the machine as it comes to temperature, and the internal fan will begin to hum consistently. Preheating ensures the brussels sprouts meet hot air immediately, which promotes rapid surface browning and prevents steaming. If you skip preheating, the sprouts may cook unevenly and lack that blistered texture. A common error is overcrowding the fryer right away, which reduces airflow and prevents crisping.
In a mixing bowl, add the Brussels sprouts, then add the oil and mix well. Add the garlic powder, smoked paprika, salt, and pepper and mix until combined.: You will feel the slight resistance as the oil coats each halved or whole sprout, and the spices will cling to the oiled surfaces. This coating is critical because it creates the Maillard reaction on the outer leaves while seasoning the interior. If the sprouts are too wet they will steam rather than brown, so ensure they are dry before adding oil. Mixing thoroughly prevents pockets of over seasoned or under seasoned pieces, which is a frequent misstep.
Add the seasoned Brussels sprouts to an air fryer basket and air fry for 10 minutes, tossing halfway through.: Early on you will hear a soft sizzle as moisture evaporates, followed by a lighter, crisping hiss as sugars caramelize. At the halfway point the edges will darken and some outer leaves may flutter free. Tossing ensures even exposure to hot air and consistent color development across pieces. Avoid leaving them unturned for the entire time, which can lead to overly charred spots on one side and undercooked centers on the other.
Transfer into a bowl and serve immediately.: When you move them out of the basket, the aroma will be toasty with a hint of smoked spice, and the color will range from golden to deep brown on the edges. Serving right away preserves the crisp exterior because steam has not had time to soften the texture. If you let them sit covered, condensation forms and sogginess sets in, which is a common disappointment. For optional finishing, sprinkle the parmesan cheese now so it warms and clings just slightly to the hot surfaces.
