To make these yakitori, start by soaking your bamboo skewers in water for at least 30 minutes. This prevents them from burning when they go into the hot skillet and oven. While the skewers are soaking, preheat your oven to 400°F.
Next, make the marinade. Add the dark soy sauce, rice vinegar, mirin, oyster sauce, minced garlic, honey, sesame oil, and white pepper to a bowl and whisk until fully combined. Add the chicken pieces to the bowl and toss to coat. Allow the chicken to marinate for at least 15 minutes.
While the chicken is marinating, cut the green onions into 1½" pieces and set them aside. Once the chicken has marinated, thread the chicken and green onion pieces onto the soaked skewers, alternating between the two.
Make the glaze by whisking together the dark soy sauce and honey in a small bowl, then set it aside.
Heat a large cast iron skillet over high heat and add the avocado oil. You want the skillet to be very hot before adding the skewers. Once the oil is shimmering, add the skewers to the skillet. Do not touch them for 5-6 minutes, until they release easily from the pan and have developed a nice sear. Flip them over and allow them to cook undisturbed for another 5 minutes on the other side. Once both sides have a good char, brush the skewers generously with the glaze.
Carefully transfer the cast iron skillet to the preheated oven and bake for 7-8 minutes, or until the chicken is fully cooked through and reaches an internal temperature of 165°F. Remove the skillet from the oven, brush the skewers with another round of glaze, and garnish with sesame seeds. Serve immediately and enjoy!