In a small mixing bowl, whisk together the ¼ cup soy sauce, ¼ cup mirin, 2 tbsp sake, and 2 tbsp brown sugar until the sugar dissolves. In a separate tiny mixing bowl, mix the 1 tsp cornstarch and 1 tbsp water until smooth to make a slurry. Set both aside.

Pat the 1 ½ lb boneless skinless chicken thighs patted dry thoroughly dry with paper towels — this is key for a good sear. Season both sides evenly with ½ tsp kosher salt and ¼ tsp black pepper freshly ground.

Heat a large skillet over medium-high heat and add the 1 tbsp neutral oil such as vegetable. Once the oil is shimmering, add the 1 ½ lb boneless skinless chicken thighs patted dry in a single layer, smooth side down. Do not crowd the pan — cook in batches if needed.

Sear the 1 ½ lb boneless skinless chicken thighs patted dry undisturbed for 5–6 minutes, until the bottom is deep golden brown. Flip and cook for another 4–5 minutes, until the internal temperature reaches 165°F. Transfer the 1 ½ lb boneless skinless chicken thighs patted dry to a plate and tent loosely with aluminum foil.

Reduce the heat to medium. Pour the ¼ cup soy sauce mixture into the same skillet, scraping up any browned bits from the bottom with a wooden spoon. Bring to a gentle simmer.

Give the 1 tsp cornstarch slurry a quick stir and pour it into the simmering sauce. Whisk constantly for 1–2 minutes until the sauce thickens to a glossy, syrupy glaze that coats the back of a spoon.

Return the 1 ½ lb boneless skinless chicken thighs patted dry (and any resting juices) to the skillet. Turn the pieces to coat them thoroughly in the glaze, cooking for 1 more minute so the 1 ½ lb boneless skinless chicken thighs patted dry is heated through and well-coated.

Slice or serve the 1 ½ lb boneless skinless chicken thighs patted dry whole over steamed 2 cups white rice uncooked. Spoon any remaining sauce from the pan over the top. Garnish with sliced stalks green onions thinly sliced and toasted 1 tsp sesame seeds toasted.