In a mixing bowl, combine soy sauce, olive oil, minced garlic, lime juice, cumin, and chili powder. Mix well.

Place the flank steak in the marinade, ensuring it is well coated. Let it marinate for at least 15 minutes.

Preheat the grill to medium-high heat. Remove the steak from the marinade and grill for about 4-5 minutes on each side, or until desired doneness.

While the steak is cooking, prepare the rice and other bowl components. In each bowl, layer cooked rice, black beans, corn, cherry tomatoes, lettuce, and avocado.

Once the steak is cooked, let it rest for a few minutes before slicing it thinly against the grain. Top each bowl with sliced steak, cilantro, and a dollop of sour cream if desired.