Pat the chicken dry and trim excess skin.
Whisk buttermilk, hot sauce, 1 1/4 tsp salt, and 1 tsp pepper.
Submerge chicken in the buttermilk. Chill 30 minutes.
Stir flour, cornstarch, paprika, garlic, onion, cayenne, thyme, baking powder, 1 1/4 tsp salt, and 1 tsp pepper.
Set a wire rack over a sheet pan for dredged pieces.
Lift chicken from marinade, let drip, then press into seasoned flour.
Rest coated chicken on the rack 10 minutes to set the crust.
Heat oil in a deep pot to 350°F, about 2 inches deep.
Fry in batches 12–14 minutes, turning, until deep golden and 165°F inside.
Drain on a clean rack. Sprinkle lightly with salt and rest 5 minutes.