Preheat oven to 400°F (200°C). Line a baking sheet with foil and place a wire rack on top. Lightly coat the rack with cooking spray.

In a large bowl, pour buttermilk over the chicken thighs, ensuring each piece is coated. Let sit for 15 minutes.

In another bowl, combine flour, Parmesan cheese, garlic powder, onion powder, paprika, salt, black pepper, and cayenne pepper (if using).

Remove each chicken thigh from the buttermilk, allowing excess to drip off. Dredge in the flour mixture, pressing lightly to adhere.

Place the coated chicken thighs on the prepared rack. Lightly spray the tops with cooking spray.

Bake for 35-40 minutes, or until the chicken is golden brown and the internal temperature reaches 165°F (74°C).