In a large bowl, whisk the buttermilk, pickle brine, hot sauce, paprika, cayenne pepper, garlic powder, and black pepper to make the marinade. Add the chicken tenders, cover the bowl, and marinate for at least 2 hours.
In a shallow bowl, mix the flour, cornstarch, paprika, cayenne pepper, onion powder, garlic powder, salt, and black pepper.
Remove the chicken tenders from the marinade. Dredge each tender in the flour mixture until evenly coated, then set aside. Repeat with the remaining chicken tenders.
Heat the oil in a large, deep pot over medium heat until it reaches 350°F. Carefully add the chicken tenders and fry for 5 to 6 minutes, until the chicken is cooked through and the coating is crisp and golden.
Add ½ cup of the hot oil reserved from frying the chicken, brown sugar, cayenne pepper, paprika, and garlic powder to a small bowl. Whisk until well combined to make the Nashville hot sauce.
Brush the Nashville hot sauce over the fried chicken tenders. Alternatively, pour the sauce over the tenders and toss until evenly coated.
Serve the Nashville hot chicken tenders topped with crumbled feta cheese, if desired, and ranch for dipping.