Trim and Season Chicken: Place the 4 chicken thighs on a cutting board and use a sharp chef's knife to trim any excess fat overhanging the edges, leaving the natural marbling intact for flavor. Pat each thigh completely dry with paper towels, pressing firmly on both sides to remove surface moisture. Season both sides of each thigh with 0.5 teaspoon of kosher salt and 0.25 teaspoon of black pepper. Dry thighs make direct contact with the hot pan surface, which produces a deeper sear and better caramelization when the glaze is applied.
Mix the Teriyaki Glaze: Combine the 60ml / 0.25 cup of soy sauce, 60ml / 0.25 cup of mirin, 2 tablespoons of sake, 1.5 tablespoons of granulated sugar, 1 teaspoon of freshly grated ginger, and 2 minced garlic cloves in a small mixing bowl. Stir the mixture with a spoon until the sugar fully dissolves. In a separate small bowl, whisk together 1 teaspoon of cornstarch and 1 tablespoon of cold water until no lumps remain. Set both bowls aside within arm's reach of the stove, as the glaze comes together quickly once cooking begins.
Sear the First Side: Heat 1 tablespoon of avocado oil in a 12-inch stainless steel or cast iron skillet over medium-high heat for 2 minutes until the oil shimmers and a small drop of water flicked into the pan evaporates on contact. Place the seasoned chicken thighs smooth-side down in a single layer and cook for 6 minutes without pressing or moving them until the underside is deep golden brown and lifts cleanly from the pan surface.
Flip and Cook Through: Flip each chicken thigh using tongs and cook the second side for 5 minutes over medium heat until the thigh feels firm when pressed at the thickest point and an instant-read thermometer inserted into the center of the largest thigh reads 74°C / 165°F. Transfer the cooked thighs to a clean plate and leave all pan drippings in the skillet.
Build and Reduce the Glaze: Pour the prepared soy sauce mixture into the same skillet with the drippings over medium-high heat. Stir the liquid with a wooden spoon, scraping up any browned bits from the pan floor, and cook for 2 minutes until the mixture bubbles actively across the full surface. Stir the cornstarch slurry once more to recombine it, then pour it into the simmering glaze while stirring continuously. Cook for 1 additional minute until the glaze thickens to a consistency that coats the back of the spoon and leaves a clean line when you draw a finger through it.
Glaze and Rest the Chicken: Return the seared chicken thighs to the skillet and turn each one to coat all sides in the glaze over medium heat for 1 minute until the surface of each thigh takes on a glossy lacquer-like sheen. Remove the skillet from the heat and let the chicken rest in the pan for 3 minutes so the glaze sets and the juices redistribute evenly before slicing or serving.
