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 and pat dry with paper towels.

Heat the oil in a cast-iron skillet over medium-low heat. Do not use a grill pan. When the oil is hot and shimmering, add the chicken thighs to the pan in a single layer. Cook the chicken in batches; do not overcrowd the pan. Place a second pan over the chicken thighs, pressing down and flattening the meat. Cook for 3 to 4 minutes. Flip the chicken, replace the weight, and cook for another 3 to 4 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 teriyaki sauce: Combine all the ingredients 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.