In a large container, mix soy sauce, cayenne pepper, garlic powder, vinegar, sesame oil, black pepper, and sesame seeds. Add beef cubes, coat well, cover, and refrigerate overnight or at least 8 hours.

To cook, heat a grill to high or a heavy skillet until screaming hot. Cook marinated beef cubes for 1 minute per side on the grill or 2 minutes per side in the skillet until caramelized.

While the beef cooks, fry the sliced green onion tops in the same pan and add any remaining marinade for extra sauce.

Meanwhile, cook the rice according to package directions.

Serve the sizzling beef and sauce over the cooked rice, garnishing with extra sesame seeds if desired.