Preheat your oven to 375°F (190°C).

Line a 8x8-inch baking dish with aluminum foil, extending it over the edges for easy cleanup. Spray the foil with non-stick cooking spray to keep the chicken thighs from sticking to it.

In a small bowl, mix cold water and cornstarch to create a slurry and then set it aside.

In a small saucepan, combine soy sauce, brown sugar, minced garlic, rice vinegar, and ginger powder. Bring them all to a simmer over low to medium heat, stirring to dissolve the sugar.

Once simmering, slowly whisk in the cornstarch slurry. Stir the sauce frequently, until it thickens enough to coat the back of a spoon (this will take about 5-7 minutes).

Remove the teriyaki sauce from heat and let it cool slightly. Reserve about ½ cup of the sauce for basting.

Place the chicken thighs in the prepared baking dish, ensuring that they don't overlap. Brush about 2 tablespoons of teriyaki sauce on top of the chicken, coating both sides.

Bake the chicken for 35-45 minutes in the preheated oven, basting them with the reserved sauce every 10 minutes. Cook them until their internal temperature reaches 165°F (74°C) when tested with an instant-read thermometer inserted into the thickest part.

Remove the chicken from the oven and let them rest for 5 minutes. This will allow the juices to be redistributed, ensuring that your chicken is moist.

Feel free to garnish the chicken thighs with sesame seeds, and serve with steamed white rice or brown rice, vegetables (such as broccoli, bok choy, carrots, and snap peas), sliced green onions for garnish, and a side of the remaining sauce for dipping.