Preheat the air fryer to 375°F (190°C).
Trim the ends of the Brussels sprouts and remove any damaged outer leaves.
Cut any large sprouts in half for even cooking.
Place the Brussels sprouts in a medium bowl.
Add olive oil and kosher salt.
Toss until evenly coated.
Arrange the sprouts in a single layer in the air fryer basket.
Cook for approximately 15 minutes, shaking the basket halfway through cooking.
The Air Fryer Brussels Sprouts are done when they are golden brown and tender inside.
While the sprouts cook, prepare the optional shallot butter.
Melt the butter in a microwave-safe bowl or small saucepan.
Stir in the finely chopped shallot and red wine vinegar.
When the Brussels sprouts finish cooking, transfer them to the bowl with the shallot butter.
Toss gently until coated.
Serve immediately while hot and crispy.