Whisk 1 Tbsp. salt, 2 tsp. black pepper, paprika, cayenne, garlic powder, and onion powder in a small bowl.

Season chicken with spices.

Place chicken in a medium bowl, cover, and chill overnight.

Let chicken stand covered at room temperature for 1 hour.

Whisk buttermilk, egg, and 1/2 cup water in a medium bowl.

Whisk flour, cornstarch, remaining 1 Tbsp. salt, and remaining 1 Tbsp. pepper in a 9x13x2″ baking dish.

Pour oil into a 10″–12″ cast-iron skillet or other heavy straight-sided skillet (not nonstick) to a depth of 3/4″.

Prop deep-fry thermometer in oil so bulb is submerged.

Heat over medium-high heat until thermometer registers 350°.

Meanwhile, set a wire rack inside a large rimmed baking sheet.

Working with 1 piece at a time (use 1 hand for wet ingredients and the other for dry ingredients), dip chicken in buttermilk mixture, allowing excess to drip back into bowl.

Dredge in flour mixture; tap against bowl to shake off excess.

Place 5 pieces of chicken in skillet.

Fry chicken, turning with tongs every 1–2 minutes and adjusting heat to maintain a steady temperature of 300°–325°, until skin is deep golden brown and an instant-read thermometer inserted into thickest part of chicken registers 165°, about 10 minutes for wings and 12 minutes for thighs, legs, and breasts.

Using tongs, remove chicken from skillet, allowing excess oil to drip back into skillet.

Transfer chicken to prepared rack.

Repeat with remaining chicken pieces.

Let cool for at least 10 minutes before serving.