Pick firm Brussels sprouts with minimal yellowing, trim the stem ends, remove any tired outer leaves, and slice each sprout in half lengthwise.
If some sprouts are very large, quarter them so they cook more evenly.
Rinse if needed, then dry the sprouts very well with a towel.
Toss the sprouts in a bowl with olive oil, salt, black pepper, garlic powder, and optional smoked paprika.
Preheat the air fryer to 375°F for 3 minutes if your model allows it.
Arrange the sprouts in a single layer, cut side down where possible.
Air fry for 8 minutes, then shake the basket well.
Cook 4 to 8 minutes more, until the edges are deeply browned and the thickest sprout is tender.
For extra crunch, raise the heat to 400°F for the last 2 minutes.
Finish as desired: serve plain, toss with 1 to 2 tablespoons balsamic glaze, or toss with Parmesan while hot.
For a thicker parmesan crust, add the cheese and air fry 1 minute more, watching closely.