Clean the Brussels sprouts under running water to remove any dirt. Chop them into half lengthwise and trim the bottom stem. Remove and discard any yellow or discolored leaves. Note: You can use really small sprouts as is without chopping them.
Dry the Brussels sprouts using a clean dish towel or paper towel.
Transfer the Brussels sprouts to a medium-sized bowl.
Add oil, salt, pepper, and garlic powder over the Brussels sprouts. Toss to combine.
Preheat the air fryer and set it to 350°F.
[Optional step] Line the air frying tray with parchment paper.
Transfer the Brussels sprouts over the parchment paper – don't overcrowd them. Depending on the size of your airfryer, you may have to make them in batches.
Place the air fryer tray in the air fryer and cook at 350°F. Set the timer for 15 minutes.[Note: It could take about 20 minutes, depending on your model]
Stir around the 8-minute mark and continue to cook till Brussels sprouts have nicely browned and turned crisp. [Note: If using an oven-style air fryer, rotate the tray for even cooking.
Transfer the Brussels sprouts to serving plates. Serve warm sprinkled with parmesan cheese.