Marinate the Chicken:
In a shallow dish, whisk together soy sauce, water, sugar, garlic powder, and white pepper. Add the chicken thighs and coat them well in the marinade. Cover and refrigerate for at least 30 minutes or up to 1 hour.

Prepare the Chicken:
Remove the chicken from the marinade and pat dry using paper towels. Heat canola oil in a heavy-bottomed skillet (preferably cast iron) over medium-low heat.

Once the oil is hot and shimmering, add the chicken thighs in a single layer. If needed, cook in batches to avoid overcrowding. Use a second pan or weight to press down on the chicken as it cooks. Sear for 3 to 4 minutes on each side, pressing down again after flipping, until the chicken is fully cooked and golden brown. Internal temperature should reach 170°F.

Make the Teriyaki Sauce:
While the chicken is cooking, whisk together all the sauce ingredients in a small saucepan until smooth. Place the pan over medium heat and bring to a simmer. Continue to stir and cook for 1 to 2 minutes, or until the sauce thickens.

Serve:
Slice the cooked chicken and drizzle with the warm teriyaki sauce. Serve with steamed rice or vegetables for a complete meal.