Step 1
Whisk 2 cups buttermilk, 1½ tsp. smoked paprika, ¼ tsp. cayenne pepper, 1 tsp. Diamond Crystal or ½ tsp. Morton kosher salt, and 1 tsp. garlic powder in a shallow bowl; season with freshly floor black pepper. Add 4 4–6-oz. catfish fillets, patted dry, to marinade and switch to coat. Let sit at room temperature, turning often, 10–quarter-hour.
Step 2
In the meantime, stir ¾ cup finely chopped pickles, ½ cup mayonnaise, 2 Tbsp. entire grain mustard, 1 Tbsp. finely chopped chives, 1 Tbsp. recent lemon juice, ½ tsp. Diamond Crystal or ¼ tsp. Morton kosher salt, and remaining 1 tsp. garlic powder in a small bowl to mix; season with black pepper. Set rémoulade apart.
Step 3
Whisk ½ cup medium-grind cornmeal, ⅓ cup all-purpose flour, and 1½ tsp. Diamond Crystal or 1 tsp. Morton kosher salt in one other shallow bowl.
Step 4
Working with 1 fillet at a time, elevate fish from marinade, letting extra drip again into bowl. Dredge in cornmeal combination, urgent firmly to stick. Switch fish to a baking sheet.
Step 5
Pour 3–4 cups vegetable oil into a big cast-iron skillet to come back ½” up sides; match with thermometer. Warmth over medium-high till thermometer registers 350°.
Step 6
Working in 2 batches and returning oil to 350° between batches, fry fish till golden brown, about 2 minutes per facet. Switch to a wire rack set inside a clear rimmed baking sheet and instantly season with salt.
Step 7
Serve fish with ketchup and reserved rémoulade.
Do Forward: Rémoulade may be made 1 day forward. Cowl and chill.
