In a small saucepan over medium heat, add soy sauce, mirin, sake, brown sugar, ponzu, ginger, and garlic. Stir well and bring it to a boil.
Reduce heat and simmer until the sauce thickens, about 10 minutes. Remove from the heat and let it cool to room temperature, about 45 to 60 minutes.
In a bowl, add chicken cubes and pour the sauce over, reserving 1/3 cup for basting the skewers later. Stir well, cover and chill for at least 4 hours but preferably overnight.
Next day, stir the chicken cubes before skewering them, leaving a little gap between each.
Bring barbecue temperature between 500ºF to 550ºF and generously coat the grates with canola oil.
Place skewers on the bbq, close the lid and grill 4 minutes before flipping.
Baste with the reserved sauce and cook for another 4 minutes or until the internal temperature reaches 165ºF.
Baste once again before removing the skewers from the heat. Serve them on a bed of rice and garnish with sesame seeds plus chopped green onions.