Whisk buttermilk, hot sauce, salt, and pepper in a large bowl. Add chicken and coat well.
Marinate at least 20 minutes at room temp, or refrigerate up to 8 hours for deeper flavor.
Set a wire rack over a sheet pan. Stir flour, cornstarch, salt, paprika, garlic powder, onion powder, cayenne, and baking powder in a shallow dish.
Heat oil in a heavy pot to 330°F over medium heat. Maintain between 325°F and 350°F.
Lift chicken from marinade, letting excess drip. Dredge in the flour mixture, pressing to help it cling.
Rest dredged chicken on the rack for 10 minutes so the crust sets.
Fry chicken in batches 12–15 minutes, turning as needed, until deep golden and 165°F at the thickest part.
Drain on a clean rack. Sprinkle with a pinch of salt and rest 5 minutes before serving.