Put the cut chicken into a large bowl and season it with kosher salt, garlic powder, and onion powder; toss well to coat all pieces evenly.
Pour the buttermilk and hot sauce over the chicken, mix thoroughly, then cover and refrigerate to marinate for at least 2 hours, ideally overnight.
Combine all-purpose flour, cornstarch, kosher salt, paprika, and black pepper in a separate bowl and whisk until blended.
Heat peanut oil in a deep fryer or heavy skillet to 325°F, reducing to 300°F if the chicken browns too fast.
Remove chicken pieces one by one from the marinade, letting extra buttermilk drip off, then dredge thoroughly in the flour mixture; shake off excess coating and let them sit a few minutes until the batter firms up.
Fry chicken in small batches of three or four pieces, turning occasionally, until a deep golden color forms and the internal temperature hits 165°F; dark meat generally takes 12-14 minutes, white meat 8-10 minutes.
Transfer cooked chicken to a paper towel-lined tray and sprinkle immediately with flaked salt if desired; repeat until all chicken is fried.
