In a small bowl, whisk together soy sauce, brown sugar, sesame oil, minced garlic, grated ginger, black pepper, and red pepper flakes until fully combined. Set aside.

Heat a large skillet over medium-high heat and add vegetable oil. Once shimmering, add ground beef and cook for 5-7 minutes until browned.

Drain excess fat from skillet, then pour marinade over beef and stir. Cook for an additional 2-3 minutes until heated through.

Stir in sliced green onions into the beef mixture, reserving some for garnish. Cook for an extra minute.

Prepare to serve by placing butter lettuce leaves in bowls, adding a scoop of rice, and spooning the beef mixture over it.

Garnish with reserved green onions, shredded carrots, and cucumber ribbons. Serve immediately.