Submerge wooden skewers in a large cup or shallow pan of water to soak for at least 30 minutes.
Combine soy sauce, mirin, sake, water, brown sugar, garlic, ginger, and the green part of 2 green onions in a medium saucepan on the stovetop over high heat. Bring to a boil, then reduce the heat to medium and simmer for about 15-20 minutes until reduced by about half and slightly thickened. It should continue to thicken as it cools. The sauce can be refrigerated for 2-3 months.
1 cup soy sauce, 1 cup mirin, 1/2 cup sake, 1/4 cup water, 1/4 cup brown sugar, 2 cloves garlic, 1 teaspoon grated fresh ginger, 2 bunches green onions
Slice the chicken into 1 1/2-inch pieces, trimming away excess fat if necessary. Slice green onions into 1 1/2-inch pieces.
2 pounds boneless, skinless chicken thigh meat
Alternately thread pieces of chicken with pieces of green onion onto skewers that have been soaked in water for 30-60 minutes. You will likely need to fold the chicken a bit if using chicken thigh. Don't be afraid to load the skewers up pretty generously. Each skewer should hold 4 pieces of chicken and 3 green onion pieces. Season with kosher salt.
Kosher salt
Prepare a charcoal grill for medium-high heat or preheat the broiler. Set aside 1/3 of the yakitori sauce to avoid contamination. Grill over direct heat or broil for 4-5 minutes, then flip and brush each skewer with some of the yakitori sauce. Cook another 4-5 minutes on the other side, then flip and brush with sauce again, cooking another 1-2 minutes until the chicken is cooked through and the sauce is caramelized.
Transfer the finished skewers to a plate and brush with some of the reserved sauce before serving. You could also garnish by sprinkling the skewers with sesame seeds or thinly sliced green onions before serving.
Scallions