Preheat the air fryer to 400 F and spray the basket with cooking spray.
Combine the flour, cornstarch, garlic, onion powder, smoked paprika, and salt in a large bowl.
Stir the water into the dry ingredients to make a smooth batter.
Add the cauliflower to the batter and stir until well coated.
Remove the cauliflower from the batter, allowing the excess to drip back into the bowl.
Arrange the cauliflower in a single layer in the air fryer basket with a little space between each piece, working in batches as necessary.
Cook for 10 minutes, then turn the pieces over and brush the tops with Buffalo sauce.
Cook for another 10 minutes, then turn the pieces over once more to brush the other side with Buffalo sauce.
Cook the cauliflower for 3 more minutes to set the sauce.
Divide the cauliflower among the taco shells, then add in the lettuce, tomato, and scallions. Drizzle with ranch dressing, if using, and serve right away.