In one bowl, whisk together buttermilk, egg, and hot sauce.
In separate bowl, whisk together flour, cornstarch, garlic powder, onion powder, smoked paprika, cayenne pepper, and kosher salt.
In dutch oven or cast-iron skillet, heat approximately 1" of oil to 350° Fahrenheit. Place wire cooling rack over thick layer of paper towels.
One by one, dip each piece of chicken completely in buttermilk mixture, then dredge through flour mixture, fully coating all sides of chicken pieces.
Place 3-4 pieces of chicken in hot oil, but don't overcrowd the dutch oven or skillet. Cook 15 minutes, turning chicken every 2 minutes, until breading is golden brown and chicken reaches internal temperature of 160-162° Fahrenheit.
Use tongs to transfer fried chicken to wire cooling rack. Chicken will come to temperature as it rests. Repeat until all chicken pieces have been fried. Serve warm.