Whisk buttermilk and hot sauce in a large bowl. Season with some salt, pepper, garlic powder, onion powder, and paprika.
Submerge chicken in the marinade. Cover and chill at least 4 hours, preferably overnight.
Stir flour, cornstarch, baking powder, remaining salt, pepper, garlic powder, onion powder, paprika, and thyme in a shallow dish.
Lift chicken from marinade, letting excess drip. Pat lightly so a thin layer remains on the surface.
Dredge each piece in the flour mixture. Press firmly so the coating adheres. Set on a wire rack for 10 minutes.
Heat oil in a heavy pot to 325–350°F. Maintain at least 2 inches of oil depth.
Fry chicken in batches without crowding, turning as needed, until deep golden and crisp, 12–16 minutes total.
Confirm doneness at 165°F in the thickest part. Adjust heat to keep oil in range between batches.
Transfer to a clean rack to drain. Sprinkle a pinch of salt while hot, if desired.
Rest 5 minutes before serving so juices settle and crust stays crisp.