Temperature control is essential: use a reliable deep-fry thermometer. If the oil is too cold, the chicken will be greasy and soggy. If the oil is too hot, the crust will burn before the inside is cooked through.
The advantage of buttermilk: The mild acidity in buttermilk tenderizes the chicken, resulting in an incredibly juicy final product. If you don’t have buttermilk, use the milk and vinegar substitute mentioned above.
Cornstarch for crispiness: Adding cornstarch to the flour mixture inhibits the development of gluten. This creates a lighter, extra crispy crust that stays crispy longer.
Storing leftovers: Store cooled leftovers in an airtight container in the refrigerator for up to 3 days. Reheat them on a rack in a preheated oven $400^\circ\text{F} (200^\circ\text{C})$ for 10-15 minutes to restore their crispiness!