Portion the fish filets. This is going to be a beer-battered fish sandwich, so if your fish portions are a little smaller than the size of your bun, it should be just fine because the batter adds extra size to the filet.
Grab two bowls or two pie plates to serve as your dredging/batter station. The size of your bowl needs to be big enough for at least one of your fish filets to lay down flat in it.
Flour dredge: add the all-purpose flour, salt, cayenne pepper powder, and black pepper to one of the bowls. This is your dry flour dredge.
Crispy batter: make the batter by combining all-purpose flour, corn starch, baking soda, cayenne, paprika, salt, black pepper, and beer to the second bowl. Stir very well to incorporate. You will have to stir this a bit to get all the flour and liquid to become fully integrated into a thick pancake batter.
In a medium Dutch oven or pan add peanut or other frying oil until it is 2 or 3 inches deep in the pot/pan.
Heat the oil until it is 350 degrees F (176 C).
Add fish and fully coat each piece in the flour dredge.
Add floured fish to the bowl with the batter. Ensure there are no dry spots.
Then after the fish is coated fully in the wet batter, quickly lay it into the dry flour dredge and coat each piece fully.
Move each piece of fish carefully from the flour to the oil. Lay it in the oil away from you so that it does not splash onto you.
Fry fish for around 8 minutes, flipping the fish a few times to make sure it gets fully golden brown on both sides.
Once it is golden brown and cooked, move each fried filet to a paper towel-lined plate and immediately sprinkle salt on both pieces to season.
Let fish rest for a couple of minutes at least. It will be hot. You can use this opportunity to toast your buns if you desire.
Making the sandwich: add a tablespoon or so of sauce to the bottom bun.
Top the sauce with 3 to 4 pickle slices if desired. If you want a spicier sandwich, you can use pickled jalapenos.
Add a battered fish filet on top and add more sauce to the top bun. Sandwich the sandwich with the top bun.
Serve sandwiches and enjoy.