Soak the bamboo skewers in water for at least 30 minutes to 1 hour.

In a small saucepan, whisk together the soy sauce, sake, mirin, brown sugar, minced garlic, and grated ginger. Bring to a boil, then reduce heat and simmer for 5-7 minutes. Allow to cool.

Reserve ¼ cup of the cooled marinade, then add chicken and scallions to the remaining marinade and refrigerate for 1 hour.

Thread the marinated chicken and scallions onto the soaked skewers, alternating between them.

Lightly oil the grill grates and preheat to medium-high heat. Grill skewers for 6-7 minutes per side, brushing with reserved marinade.

For broiling, preheat the broiler, line a baking sheet with foil and place skewers on a wire rack. Broil for 6 minutes, then brush with sauce and broil for another 3-4 minutes.

Serve hot with the reserved dipping sauce, garnished with sesame seeds and shichimi togarashi if desired.