In a large bowl, whisk together buttermilk, salt, black pepper, smoked paprika, garlic powder, and onion powder.
Add the chicken thighs to the buttermilk mixture, ensuring they are fully submerged. Cover and refrigerate for at least 12 hours or overnight.
Preheat the oven to 425°F (220°C). Line a large baking sheet with aluminum foil and place a wire rack on top. Spray the rack with cooking spray.
In a shallow dish, combine the flour, panko breadcrumbs, cayenne pepper, and dried thyme.
Remove each chicken thigh from the buttermilk marinade, allowing excess to drip off, then dredge thoroughly in the flour mixture, pressing to adhere the coating.
Place the coated chicken thighs on the prepared wire rack. Lightly spray the tops of the chicken with cooking spray to promote crispiness.
Bake in the preheated oven for 40 to 45 minutes, or until the chicken is golden brown and reaches an internal temperature of 165°F (74°C).
Remove from oven and let rest for 5 minutes before serving.