How to Make It
Preheat oven to 400°F. Stir together Kewpie mayo, chopped dill pickles, gochujang, mustard, and scallions in a small bowl; set aside.
Combine peppercorns, coriander seeds, and caraway seeds in a spice grinder; grind until coarsely ground, about 5 seconds.
Heat oil in a large cast-iron or ovenproof skillet over high. Add salmon fillets; cook until browned, about 1 minute and 30 seconds per side. Remove from heat.
Stir together molasses, salt, and paprika in a bowl. Brush both sides of salmon fillets with molasses mixture, and sprinkle both sides evenly with peppercorn mixture, pressing to adhere. Transfer skillet to preheated oven, and roast to desired degree of doneness, 3 to 4 minutes for medium.
Brush rye bread slices evenly with melted butter. Place on a baking sheet, and toast at 400°F until golden brown, about 5 minutes.
Spread desired amount of mayonnaise mixture evenly on 1 side of toasted rye slices. Top each of 4 rye slices with 1 salmon fillet. Top evenly with kimchi, and cover with remaining 4 rye slices. Serve immediately with any remaining mayonnaise mixture.