Cut the catfish into portions about the width and length of two fingers (human fingers, that is). Place in a resealable disposable plastic bag. Add buttermilk. Close bag, removing as much air as possible. Gently turn the bag several times to coat the fish. Marinate in the refrigerator for at least one hour and as long as four hours.
Add cornmeal, flour, garlic powder, onion powder, kosher salt, and cayenne pepper to a pie pan or similar size dish with a raised edge. Stir together dry ingredients.
In a large cast iron skillet, heat 1 1/2 to 2 inches of cooking oil to 350 degrees.
While the oil is heating, remove catfish pieces from buttermilk and dredge each piece in the breading. Set aside.
When oil is hot enough, work in batches and drop a few pieces of fish in the hot oil. Let cook for 3 to 4 minutes until golden brown on one side. Flip and repeat on the other side. Drain on a rack when the fish is cooked.
Repeat with remaining fish. Don't crowd the pan when cooking.
Serve with lemon and cocktail sauce, ketchup, Comeback Sauce, or tartar sauce.