Step 1: In a large bowl or resealable bag, combine the buttermilk and hot sauce. Add the chicken pieces, ensuring they are fully submerged. Cover and refrigerate for at least 4 hours, or preferably overnight (up to 24 hours).
Step 2: In a separate large bowl, whisk together the all-purpose flour, cornstarch, paprika, garlic powder, onion powder, cayenne pepper, oregano, thyme, salt, black pepper, and celery salt (if using). Ensure all spices are evenly distributed.
Step 3: Set up a dredging station: one bowl with the marinated chicken, and the other with the seasoned flour mix. Working with one piece of chicken at a time, remove it from the marinade, letting excess drip off.
Step 4: Dredge the chicken thoroughly in the seasoned flour, pressing the flour onto all surfaces. Dip the floured chicken back into the buttermilk marinade briefly, then dredge it again in the flour mixture, ensuring a thick, craggy coating. Place the double-dredged chicken on a wire rack and repeat with remaining pieces.
Step 5: In a large, heavy-bottomed pot or Dutch oven, pour enough oil to reach a depth of 3-4 inches. Heat the oil over medium-high heat until a deep-fry thermometer reads 325-350°F (160-175°C).
Step 6: Carefully lower 2-3 pieces of chicken into the hot oil, ensuring not to overcrowd the pot. Fry for 6-8 minutes per side, or until golden brown and cooked through, with an internal temperature of 165°F (74°C). Adjust heat as needed to maintain oil temperature.
Step 7: Once cooked, use tongs to transfer the fried chicken to a clean wire rack set over a baking sheet to drain excess oil. Allow the chicken to rest for a few minutes before serving. Repeat with remaining chicken batches.