Place chicken pieces in a large bowl. Pour buttermilk and hot sauce over the chicken. Mix to coat completely. Cover and refrigerate for at least 4 hours, or preferably overnight.
In a shallow dish, whisk together the flour, cornstarch, salt, pepper, paprika, garlic powder, cayenne pepper, and baking powder. This is your dredge mixture.
Remove chicken pieces from the buttermilk, letting excess drip off, but do not wipe them dry.
Dredge each piece thoroughly in the flour mixture, pressing the flour onto the chicken to create a thick coating.
For extra crispiness, dip the floured chicken back into the remaining buttermilk for just a second, then immediately return it to the flour mixture for a second, heavy dredge. Press again to build texture.
Heat 2-3 inches of oil in a deep, heavy-bottomed pot or Dutch oven to 325 degrees Fahrenheit. Use a thermometer to monitor the temperature.
Carefully place chicken pieces into the hot oil, ensuring you do not overcrowd the pot. Work in batches.
Fry chicken for about 6-8 minutes per side, turning occasionally, until deep golden brown and cooked through (internal temperature should reach 165 degrees Fahrenheit). Adjust heat as needed to maintain oil temperature between 300 and 325 degrees Fahrenheit.
Remove cooked chicken and place it on a wire rack set over a baking sheet to drain excess oil. Do not place directly on paper towels, as this can steam the crust and make it soggy.
Serve hot.
Notes
For the crispiest crust, use a mixture of half vegetable oil and half shortening for frying.
If using chicken breasts, cut them into even pieces to promote uniform cooking time.
If the oil temperature drops too low when adding cold chicken, the crust will absorb too much oil and become greasy.