Prepare the tartar sauce. Combine the mayonnaise with dill pickles, dill, lemon juice, and sugar. Let sit for at least 30 minutes or overnight.
Make the vinegary cabbage. Combine the vinegar with boiling water, sugar, salt, and mustard seeds, stirring until the sugar and salt have dissolved. Pour the liquid over the cabbage and let sit for 30 minutes.
Preheat frying oil to 350 degrees F.
Pat dry the cod fillets and season both sides with salt and pepper. Place ¼ cup rice flour in a bowl and dredge cod in flour, shaking off excess.
Make beer batter. Combine all-purpose flour, remaining ¼ cup rice flour, baking powder, and salt in a large bowl. Add beer, whisking just until combined. It's okay if there are some lumps in the batter.
Dunk the fish in the batter, letting the excess drip off. Carefully drop fish into frying oil and fry until golden brown, about 3-4 minutes. Remove fish with a slotted spoon and place on wire rack.
Combine the melted butter with 1-2 tablespoon cayenne (depending on how spicy you want your sandwich), brown sugar, paprika, chili powder, garlic powder, black pepper, and salt. Generously drizzle the chili butter over the fish.
Assemble the sandwiches. Spread the tartar sauce on the bottom bun, top it with a fish filet, vinegary cabbage, and the remaining bun. Serve immediately.