In a mixing bowl, combine garlic powder, rice vinegar, soy sauce, hoisin sauce, brown sugar, sesame oil, sesame seeds, and ground ginger; whisk until the sugar dissolves.
In a small bowl, stir the hot water and peanut butter until smooth, then add this mixture to the marinade and mix to combine.
Place the chicken pieces in a zip-top bag or resealable container and pour in the marinade; seal and refrigerate for at least 3 hours.
Thread marinated chicken onto skewers, alternating with red and green bell pepper chunks.
Preheat a grill or grill pan to medium-high. Grill the skewers 5–7 minutes per side, or until the chicken is cooked through and no longer pink.
Optional: while the chicken cooks, bring the reserved marinade to a boil in a saucepan for 4–5 minutes, then simmer briefly; use to baste or drizzle over cooked skewers.