Pat the chicken dry, season with 1 tsp salt and ¼ tsp black pepper, whisk together the 240 ml buttermilk and 1 egg in a bowl, add the chicken, and soak for at least 30 minutes at room temperature.

Combine the 300 g flour, 60 g cornstarch, 2 tsp salt, 1 tsp black pepper, 1 tsp smoked paprika, 1 tsp garlic powder, 1 tsp onion powder, 1 tsp oregano, ½ tsp celery salt, ½ tsp white pepper, ½ tsp mustard powder, and ½ tsp cayenne in a wide, shallow dish and whisk for 30 seconds.

Lift each chicken piece from the buttermilk, press firmly into the flour mixture, dip back into the buttermilk, then press into the flour a second time, and place on a wire rack.

Heat neutral oil in a deep heavy pot to 175°C (350°F), fry chicken in batches of 3 to 4 pieces for 14 to 16 minutes (drumsticks and thighs) or 16 to 18 minutes (breasts), turning once, until the internal temperature reaches 74°C (165°F).

Transfer to a wire rack over a baking sheet, sprinkle with flaky salt, and serve immediately on a platter.