I like to leave the Brussels sprouts whole, but if you have any particularly large ones (around 2-inches thick) you may want to cut them in half to promote even cooking. For extra-crispy Brussels sprouts that aren't particularly soft in the center, you can cut them all in half. Toss the sprouts in the olive oil, and season generously with fine sea salt. (You can use up to 1/2 teaspoon-- make sure you rub it into the oiled veggies so that it is evenly distributed.)
Pour the Brussels into the air fryer tray, and shake to distribute them into a single layer. Cook at 360ºF for 15 minutes. When the cooking time is up, remove any crispy leaves that are done.
Test one of the Brussels sprouts for tenderness by piercing it with a fork, or cut one in half. The insides should be very tender, like they will melt in your mouth, with a very crispy outside. If they are not done yet, return them to the air fryer to cook for 5 more minutes.
Serve warm. These are best enjoyed right away, but you can store any leftovers in the fridge for up to 5 days. (They will loose their crispiness as they cool, but they can still be added to salads and stir-fries later.)
Video
Notes
If you have the Cuisinart Air Fryer, it tends to run hot-- up to 50ºF hotter than a basket air fryer. So, you may want to cook these at 325ºF in that particular model.