Preheat your oven to 425°F (220°C). Line a baking sheet with parchment paper and lightly grease it with a little vegetable oil.
In a large bowl, combine the buttermilk, garlic powder, onion powder, paprika, and a pinch of salt. Add the chicken thighs, ensuring they are fully submerged. Cover with plastic wrap and refrigerate for at least 1 hour (or up to overnight for more flavor).
In another bowl, mix the flour, salt, black pepper, and cayenne pepper (if using). Remove the chicken from the buttermilk, allowing excess to drip off, and then dredge each piece in the flour mixture, pressing down to adhere well.
Heat the vegetable oil in a large skillet over medium-high heat. Once hot, add the chicken pieces skin-side down and sear for about 3-4 minutes until golden brown. Flip and sear the other side for the same amount of time.
Transfer the chicken to the prepared baking sheet, skin-side up. Bake in the preheated oven for 25-30 minutes, or until the chicken reaches an internal temperature of 165°F (74°C) and is crispy.
Let the chicken rest for 5 minutes before serving.