Season the flank steak with soy sauce, fish sauce, brown sugar, lime juice, garlic powder, and black pepper. Let it marinate for at least 30 minutes, or up to 2 hours for more flavor.
Preheat a grill or grill pan over medium-high heat. Brush the grill with vegetable oil to prevent sticking.
Remove the steak from the marinade and grill for about 4-5 minutes on each side for medium-rare, or until your desired doneness is reached. Let the steak rest for 5 minutes before slicing it thinly against the grain.
While the steak is resting, prepare the Nam Jim Jaew sauce by mixing together fish sauce, lime juice, rice powder, brown sugar, chili flakes, minced garlic, and chopped cilantro in a bowl.
Serve the sliced steak with the Nam Jim Jaew sauce drizzled over the top or on the side for dipping.