Trim stems from Brussels sprouts and cut in half (or quarters if extra large).
Place Brussels sprouts in a large bowl and toss with olive oil. (You can also season with salt and pepper. If you do, adjust the amounts of salt and pepper in the Bang Bang sauce.)
Spread Brussels sprouts in a single layer in the basket of your air fryer (if you have too many for your air fryer, cook in batches).
Cook at 400°F for 6-8 minutes. Toss the Brussels sprouts and continue cooking for an additional 4-5 minutes.
Meanwhile, prepare the Bang Bang sauce by whisking together the salt, pepper, sweet chili sauce, Sriracha, lemon juice, mayonnaise, and garlic salt. Add a pinch of red pepper flakes, if desired.
Spoon ½ cup of the sauce over the Brussels sprouts and toss to coat.
Return to the air fryer and cook an additional 2-3 minutes, until the sauce is caramelized and the sprouts are toasty and tender.
Serve with remaining sauce.