Prepare the Sauce

In a medium-sized Dutch oven or a large, high-sided skillet, combine the shoyu soy sauce, brown sugar, chopped garlic, grated ginger, and 1 1/2 cups of chicken stock. Whisk the mixture thoroughly until the sugar is fully dissolved, which should take approximately 30 seconds.

Cook the Chicken

Position the chicken thighs, skin-side down, in the prepared sauce and bring the mixture to a boil over medium-high heat. Once boiling, reduce the heat to medium-low, cover the pot with a lid, and allow the chicken to simmer. Turn the chicken halfway through cooking to ensure even tenderness. Continue simmering until the chicken reaches an internal temperature of 165°F, which should take 25 to 30 minutes. Remove the pot from the heat once done.

Broil for Crispy Skin (Optional)

Carefully transfer the chicken to a rimmed baking sheet, arranging it skin-side up. For a crisp, golden skin, preheat the oven’s broiler and place the rack approximately 6 inches from the heat source. Broil the chicken for about 3 minutes, or until the skin is browned and crisp.

Thicken the Sauce

In a small bowl, whisk together the remaining 1/4 cup of chicken stock and the cornstarch until a smooth mixture forms. Gradually pour this into the sauce left in the Dutch oven, stirring to combine. Heat the sauce over medium heat, bringing it to a gentle boil. Stir continuously until the sauce thickens enough to coat the back of a spoon, which should take around 1 minute.