Three hours before the cook, combine the soy sauce, mirin, sake, sugar, garlic and ginger in a small sauce pot. Simmer for 5 minutes and remove from the heat. Allow to cool to room temperature. Reserve ¼ cup for dipping sauce.
Soak 6 wooden skewers in water.
Place the chicken in a large bowl and toss with the remaining sauce to evenly coat. Allow the chicken to marinate for 1-3 hours.
Set the EGG for direct cooking without a convEGGtor at 400°F/204°C.
Remove the chicken from the bowl and thread evenly on the skewers. Grill for 6 minutes, turning once, until the chicken reaches an internal temperature of 165°F. Transfer the chicken to a platter, garnish with scallions, sesame seeds, and serrano. Serve with the reserved sauce for dipping along with a side of baby bok choy.