Preheat the oven to 425°F. Line a baking sheet with aluminum foil and place a wire rack on top.
In a large bowl, combine buttermilk, garlic powder, onion powder, smoked paprika, salt, and black pepper. Add the chicken thighs, ensuring they are fully submerged. Cover and refrigerate for at least 1 hour, or overnight for best results.
In another bowl, mix together the flour, cornmeal, baking powder, and cayenne pepper (if using). Remove the chicken from the buttermilk mixture, allowing excess to drip off, then dredge each thigh in the flour mixture, pressing lightly to adhere.
Place the coated chicken thighs on the wire rack. Drizzle the vegetable oil over the chicken, ensuring even coverage.
Bake in the preheated oven for 35-40 minutes, or until the chicken is golden brown and reaches an internal temperature of 165°F. For extra crispiness, broil on high for an additional 2-3 minutes, watching closely to prevent burning.
Let the chicken rest for 5 minutes before serving.