In a medium bowl, whisk together soy sauce, gochujang, brown sugar, sesame oil, garlic, ginger, rice vinegar, and black pepper until sugar dissolves.

Place thinly sliced beef in a zip-top bag or shallow dish, pour marinade over, and massage to coat. Let sit at room temperature for 10 minutes while you prep the slaw.

In a bowl, combine chopped kimchi, shredded green cabbage, mayonnaise, kimchi brine, sugar, and sliced scallion. Stir well and set aside to let flavors meld.

Heat a large skillet or grill pan over high heat. Working in batches, cook beef strips in a single layer for 1-2 minutes per side until deeply caramelized and charred at the edges. Transfer to a plate and repeat with remaining beef.

While beef rests, warm tortillas in a dry skillet or directly over a gas flame for about 20 seconds per side until pliable and speckled.

Place a generous pile of beef onto each tortilla, top with kimchi slaw, a drizzle of sriracha mayo, and a sprinkle of sesame seeds and cilantro. Serve immediately.