Soak bamboo skewers in water for at least 30 minutes; this prevents burning and ensures even cooking.

In a small saucepan, combine soy sauce, mirin, sake, and sugar to create the glaze. Add garlic and ginger if using. Heat until sugar dissolves, then simmer for 5-6 minutes until slightly thickened.

Cut chicken thighs into bite-sized chunks and thread onto soaked skewers, leaving space for heat circulation.

Preheat grill over medium-high heat and lightly oil the grates.

Place skewers on the grill and cook for 3-4 minutes on one side. Brush with glaze, flip, and brush the other side before grilling for another 3-4 minutes.

Baste and flip skewers a few more times for a rich glaze and flavor, watching closely to prevent burning.

Remove skewers once cooked through (internal temperature of 165°F) and serve hot with steamed rice and garnishes.