Step 1

In a medium bowl, combine cornstarch with soy sauce. Thoroughly dissolve the cornstarch, whisking until any bits of white cornstarch no longer visible. Add soy sauce, both sugars, mirin, garlic, ginger, and pepper. Whisk sauce until ingredients are well combined. Set aside.

Step 2

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. Pound chicken evenly to about 1/2 in. thick.

Step 3

Remove cling wrap. Using a fork, poke lots of holes all over chicken pieces.

Step 4

Place chicken in large mixing bowl or large baking pan. Whisk marinade again and pour in just enough to completely cover every chicken piece front and back – reserve remaining sauce. Cover and let sit in fridge overnight, turning chicken pieces over once while marinading. Pour unused clean marinade in an airtight container and keep chilled.

Step 5

One hour before cooking, remove chicken from fridge. Grease and preheat outdoor grill (or indoor stovetop cast iron grill) until surface is smoking hot. Remove chicken from marinade, discarding used marinade. Grill chicken on medium-high heat until just-cooked, basting generously and frequently; approximately 5 minutes per side, but actual cook time varies depending on heat of your grill.

Step 6

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.

Step 7

Slice cooked chicken into strips. Serve over your white rice, brown rice, or a bed of fresh greens.  Drizzle with teriyaki sauce. Sprinkle with chopped scallions and toasted sesame seeds, if desired.