Sauce Preparation: In a medium-sized bowl, mix together the cornstarch and soy sauce until the cornstarch has fully dissolved and there are no longer any visible bits of white. Add in the remaining ingredients for the sauce, including both types of sugar, mirin, minced garlic, grated ginger, and black pepper. Mix all of the ingredients together thoroughly. Set this mixture aside.

Drying and Pounding: Take each chicken thigh and pat dry with paper towels. Trim any excess fat. Place each chicken thigh on a cutting board, covering it with cling wrap. Use a meat mallet to pound the chicken until it is approximately 1/2 inch thick.

Marinade Preparation: Remove the cling wrap and use a fork to poke holes all over the chicken pieces to allow the marinade to soak in. Place the chicken in a large mixing bowl or baking pan and pour enough marinade over it to completely cover the chicken on both sides. Cover and refrigerate overnight, flipping the chicken pieces once. Store any unused, clean marinade in an airtight container in the refrigerator.

Cooking: An hour prior to cooking, remove the chicken from the refrigerator. Grease a grill and heat it until it is smoking hot. Remove the chicken from the marinade, discarding the used marinade. Cook the chicken on medium-high heat until it is fully cooked, basting frequently if using an outdoor grill. The cook time will vary depending on the heat of the grill, but it should take about 5 minutes per side.

Sauce Heating: While the chicken is cooking, heat up a desired amount of the remaining unused sauce in a saucepan. Boil the sauce while stirring for 1 minute until it has thickened slightly. If desired, add more sugar to taste. Set the sauce aside.

Serving: Slice the cooked chicken into strips and serve over rice, greens, or your preferred base. Drizzle with the teriyaki sauce and sprinkle with sliced scallions and toasted sesame seeds, if desired.