Rinse the frog legs under cold water and pat them very dry with paper towels. Place them in a bowl and pour the buttermilk over them. Toss to coat, cover, and refrigerate for at least 30 minutes or up to 2 hours. The buttermilk tenderizes the meat and helps the coating adhere.
In a shallow dish, whisk together the flour, cornstarch, garlic powder, cajun seasoning, salt, onion powder, and cayenne pepper until evenly blended.
Remove the frog legs from the buttermilk one at a time, letting excess drip off. Dredge each leg thoroughly in the seasoned flour mixture, pressing gently to coat all sides. Shake off any loose flour.
Preheat the air fryer to 400 degrees F for 3 minutes. Spray the basket with olive oil spray.
Arrange the frog legs in a single layer in the basket, making sure they are not overlapping. Spray the tops generously with olive oil spray.
Air fry at 400 degrees F for 12 to 14 minutes, turning them over halfway through and spraying the other side with oil. They are done when the coating is crispy and golden and the meat near the bone is white and pulls away easily.
Serve the frog legs hot with a squeeze of fresh lemon juice and a sprinkle of chopped parsley.