For the chicken
1/2 cup soy sauce
1/4 cup water
1 tablespoon sugar
1/2 teaspoon garlic powder
1/2 teaspoon white pepper
1 pound boneless skinless chicken thighs
1 tablespoon canola oil
For the teriyaki sauce
1/4 cup water
1/4 cup sugar
1/4 cup soy sauce
2-1/2 tablespoons sesame oil
1 tablespoon lemon juice
1 tablespoon cornstarch
2 teaspoons brown sugar
1 teaspoon garlic powder
1/2 teaspoon ginger powder
1/2 teaspoon salt
To make the chicken, combine the soy sauce, water, sugar, garlic powder, and white pepper in a shallow dish. Add the chicken thighs, and turn to coat. Cover the dish, and marinate for 30 minutes to 1 hour in the refrigerator.
Remove the chicken from the marinade. Pat dry with paper towels.
Preheat the oil in a cast-iron skillet over medium-low heat. Once the oil is hot and shimmering, add the chicken thighs in batches, only adding enough chicken so it cooks in a single layer. Cook the chicken for three to four minutes, until lightly browned.
Flip the chicken. Weigh down the chicken with a second pan, presing it down and flattening the meat. Cook for three to four minutes, until browned and seared.
Flip the chicken and replace the weight. Cook for another three to four minutes, until the chicken is browned and cooked through. A thermometer should measure 170°F in the thickest part of the thigh.
While the chicken is cooking, prepare the sauce by combining the water, sugar, soy sauce, sesame oil, lemon juice, cornstarch, brown sugar, garlic powder, ginger powder and salt in a small saucepan. Whisk to remove any clumps. Bring the mixture to a simmer over medium heat. Simmer until thickened, 1 to 2 minutes.
Slice the chicken and serve it with the sauce.