Let's make sauce. In a small bowl, add soy sauce, sake, mirin, sugar, and minced garlic. Mix everything.
Use a medium or large pan, and turn the heat to medium. Add a thin layer of vegetable oil, then add the beef and cook until there is no more pink.
Wipe with a paper towel to remove extra oil.
Add the sauce you just made. Mix with the beef and bring to a boil once.
Turn the heat to medium low and move the beef to the sides (Leaving an empty circle in the middle).
Add the cooked rice in the middle. Add the corn, green onion, butter, and black pepper on top.
Cover with a lid and cook for 3 minutes.
After 3 minutes, turn the heat off, then mix everything lightly.