Spicy Cajun sandwich sauce: combine all ingredients in a small mixing bowl and whisk to combine.
Store in a sealed container in the refrigerator for up to 2 weeks.
Catfish marinade: Add catfish filets to a small container or plastic zip-top bag and pour in the buttermilk and hot sauce. Mix everything to combine and add to the fridge to marinate for at least 1 hour and up to overnight.
Catfish dredge and frying: when it is time to fry, add cornmeal, flour, and Cajun seasoning to a large plate or bowl. Mix to combine everything very well.
Remove each catfish filet from the marinade, add it to the plate, and coat each piece of fish very well with the cornmeal mixture. Make sure there are no dry spots.
In a large pot or pan over medium-high heat, bring peanut or vegetable oil to 350 degrees F (176 C).
Once the oil is at the proper temperature, slowly add each catfish filet and fry for 2 to 3 minutes before flipping and frying for another 2 to 3 minutes on the second side. At this point, the fish should be golden brown and crispy and cooked all through.
Remove the fried fish to a paper towel-lined sheet pan or a cooling rack placed over a sheet pan to rest.
Sandwich assembly: while the fish rests you can toast your buns.
Once the buns are toasted, add 1 to 2 tablespoons of spicy Cajun sandwich sauce to the bottom piece of each bun. Then top the sauce with a fried catfish filet.
Then add any toppings like lettuce, tomato slices, and/or pickles. Add more sandwich sauce to the top bun if desired.
Serve and enjoy.