To begin, prepare the yakitori sauce by combining soy sauce, mirin, sake, and sugar in a small saucepan. Optionally, add minced garlic and ginger for extra depth of flavour. Bring the mixture to a boil over medium heat, stirring occasionally until the sugar dissolves. Lower the heat and simmer for 8-10 minutes until the sauce thickens slightly. Set aside and allow it to cool.

While the sauce cools, take a large bowl and place the chicken pieces inside. Lightly season the chicken with salt and pepper, ensuring each piece is evenly coated. This will help the chicken develop a nice flavour as it cooks. Prepare the green onions by cutting them into even, bite-sized pieces.

Thread the chicken and green onions alternately onto the soaked bamboo skewers. You can use two pieces of chicken followed by one piece of green onion to balance flavors. Be careful not to overcrowd the skewers; leave a little space between each piece to ensure even cooking.

Preheat your grill (or oven broiler) to medium-high heat, around 400°F (200°C). If using a broiler, arrange the rack in the top third of the oven to ensure that the skewers get a nice char.

Place the chicken skewers on the preheated grill or under the broiler. Cook for 4-5 minutes on one side until the chicken starts to brown. Avoid moving the skewers too much to ensure that they develop a good char and seal in the juices.

After 4-5 minutes, brush the yakitori sauce generously over the skewers, then turn them over to cook the other side. Grill for another 4-5 minutes, brushing more sauce as you turn the skewers, allowing the sauce to caramelize slightly for extra flavour.

Continue grilling and basting with the sauce until the chicken is fully cooked and the skin is golden and crispy, about 10-12 minutes in total. If using the broiler, keep a close eye on the skewers to prevent burning, and adjust the temperature if necessary.

Serve the yakitori skewers hot off the grill, arranged on a serving platter. Sprinkle with sesame seeds or chopped scallions for a fresh garnish. Yakitori is best enjoyed with a side of steamed rice or accompanied by cold Japanese beer for an authentic experience.