Sprinkle the catfish filets with kosher salt on both sides. Cut fillets into strips. Place the seasoned strips in a shallow dish or resealable bag and cover with the buttermilk, turning to coat. Set aside to soak while preparing the rest of the ingredients and oil, or transfer to the fridge for up to 8 hours to marinate.
Combine cornmeal, flour, seasoned salt, paprika, black pepper, and cayenne pepper in a large resealable ziplock bag. Whisk or toss to evenly disperse spices.
Heat ½ to 1 inch of oil in a large cast iron pan or skillet over medium-high heat until it reaches 350-365℉.
Remove the catfish from the buttermilk soak 1-2 pieces at a time, letting excess buttermilk drip off. Transfer the catfish pieces to the bag of seasoned cornmeal and flour, seal, and toss to coat well. Transfer to a plate and repeat with the remaining fish pieces and coating.
Carefully add catfish pieces to the hot oil, working in batches so as not to crowd the pan. Fry for 4-8 minutes, flipping halfway through, until golden brown on the outside and cooked through in the middle. The cooking time will depend on the thickness of the fillets. Be sure to watch the heat as the fish fries so you can adjust up or down to keep the oil temp in the 350 to 365℉ range as much as possible.
Transfer the fried catfish to a baking sheet lined with a wire rack and place in a warm oven (225℉) while frying the remaining catfish pieces.