Optional Garnish: Thinly sliced scallions and toasted sesame seeds
Instructions
In a medium bowl, combine cornstarch with soy sauce. Thoroughly dissolve the cornstarch, whisking until any bits of white cornstarch no longer visible.
Add both sugars, mirin, garlic, ginger, and pepper to the soy sauce mixture. Whisk sauce until ingredients are well combined. Set aside.
Thoroughly dry chicken thighs with paper towels, and trim. Working one at a time, place each piece on a cutting board and top chicken with cling wrap. chicken evenly to about 1/2 in. thick.
Remove cling wrap. Using a fork, poke holes all over chicken pieces; this allows marinade to soak in.
In a large bowl, pour in just enough marinade to completely submerge chicken; reserve unused sauce. Cover and let sit in fridge overnight, turning chicken pieces over once while marinading. Pour unused marinade in an airtight container and keep chilled.
One hour before cooking, remove chicken from fridge. Generously grease and preheat outdoor grill (or indoor cast iron grill) until oil is smoking hot. Remove chicken from marinade, discarding used marinade.
Grill chicken on medium-high heat until just-cooked. If using outdoor grill baste generously and frequently; less frequently for indoor grill. Cook approximately 5 minutes per side (actual cook time varies depending on heat of your grill.)
While chicken is cooking, pour desired amount of remaining, unused/clean sauce in a saucepan. Bring to a boil, stirring 1 minute until slightly thickened. If desired, add a bit more sugar to taste. Set aside.
Slice cooked chicken into strips. Drizzle with teriyaki sauce and garnish with chopped scallions and toasted sesame seeds, if desired.
Notes
Mirin is a Japanese sweet rice wine that can be found at Asian supermarkets, as well as regular grocery stores in their Asian or ethnic aisle.
Chicken thighs will yield the most tender, flavorful results. You can also use chicken breasts, but they tend to dry out easier during cooking.