Rinse the chicken in buttermilk with the salt, onion powder, mustard powder, and paprika for at least 2 hours.
Whisk together 2 cups of all-purpose flour, 1 cup cornstarch, 2 teaspoons kosher salt, 2 teaspoons paprika, 2 teaspoons garlic powder, and 2 teaspoons black pepper. Set aside.
Fill a large pot, deep fryer, or dutch oven with a high smoke point vegetable oil of your choice, and preheat to 325°F.
Taking the chicken pieces one at a time, let the excess buttermilk drip off. Coat in the flour mixture, making sure the chicken is well coated. Shake off any excess. Set the chicken aside and let it sit for a few minutes until the coating starts to look a little pasty.
Carefully add the piece of chicken to the deep fryer and continue with the next piece. Do not add more than three or four pieces at a time. You will need to fry in batches to avoid overcrowding the fryer. Fry until golden brown, turning every few minutes.
Chicken is done when golden brown and the internal temperature reaches 165°F.
When the chicken is done frying, place it on a paper towel-lined sheet tray and optionally sprinkle a little flaked salt on top. Continue with the rest of the chicken pieces.
