Marinate the chicken: In a medium bowl, combine soy sauce, mirin, sake, sugar, minced garlic, grated ginger, and sesame oil (if using). Add the chicken pieces and toss to coat. Cover and refrigerate for at least 30 minutes, or up to 2 hours for more flavor.

1. Prepare the skewers: Soak wooden skewers in water for 10 minutes to prevent them from burning while grilling. Thread the marinated chicken pieces onto the skewers, alternating with vegetables like bell pepper and onion if desired.
2. Grill the chicken: Preheat a grill or grill pan over medium-high heat. Lightly brush the grill with vegetable oil. Grill the chicken skewers for 5-7 minutes per side, or until the chicken is cooked through and has a slightly charred exterior. Brush the skewers with the marinade during grilling for extra flavor.
3. Make the tare sauce (optional): In a small saucepan, combine soy sauce, mirin, sugar, and sake. Bring to a simmer over medium heat and cook for 3-5 minutes, stirring occasionally, until the sauce thickens slightly. If you prefer a thicker sauce, mix cornstarch with a small amount of water and add it to the simmering sauce. Once the sauce is ready, remove from heat.
4. Serve: Arrange the grilled chicken skewers on a platter. Drizzle the tare sauce over the top if desired, and garnish with sesame seeds or chopped green onions for added flavor.