Prepare the Buttermilk Marinade: In a large bowl, whisk together buttermilk, salt, garlic powder, smoked paprika, cayenne pepper, and black pepper until well combined.
Marinate the Chicken: Add the chicken tenders to the bowl. Make sure every piece is coated in the seasoned buttermilk mixture then cover and refrigerate for at least 4 hours, or up to 24 hours.
Prepare the Breading: In a shallow dish, combine the flour, baking powder, salt, paprika, black pepper, garlic powder, onion powder, dried parsley, smoked paprika, and cayenne pepper. Add 2 tablespoons of buttermilk to the flour mixture. Use your fingertips to rub the buttermilk into the flour to form small clumps (when fried, these bits create an ultra-crispy, craggy texture).
Dredge the Chicken: Remove the chicken tenders from the marinade a few at a time, allowing any excess to drip off. Coat the chicken tenders in the seasoned breading mixture, pressing firmly to ensure the breading sticks.
Set the breaded chicken tenders aside on a wire rack or plate.
To Fry the Chicken: Place a wire cooling rack inside a large baking sheet lined with foil. Heat about 3 inches of oil in a deep pan or 6 to 7-quart Dutch oven to 350°F (175°C). Use a deep fry thermometer for accuracy. Working in batches, carefully lower 3-4 chicken tenders at a time into the hot oil. Fry for 5–6 minutes, turning occasionally, until golden brown and cooked through. The internal temperature should reach 165°F (74°C) as measured by a digital meat thermometer. Return the oil to 350°F (175°C) and repeat until all of the chicken tenders have been cooked.
Drain and Serve: Transfer the cooked tenders to the prepared wire rack (placed over a baking sheet) to drain excess oil. Sprinkle with fresh chopped parsley if desired.