Whisk together soy sauce, brown sugar, honey, mirin, sake, garlic, and ginger until the sugar dissolves and the mixture looks uniform. Reserve half for the sauce, then toss the chicken thighs with the other half and refrigerate for 20 minutes so the surface tastes seasoned.

Heat vegetable oil in a large skillet over medium-high heat until shimmering. Add the marinated chicken thighs and cook 5-6 minutes per side until caramelized and the center reaches 165°F, with dark amber edges showing as the glaze sets.

Remove the chicken from the skillet to a plate and keep any juices behind, then lower heat slightly to avoid scorching when you add the reserved marinade.

Pour the reserved marinade into the pan and bring it to a simmer, scraping up any browned bits from the bottom. Stir in the cornstarch slurry and cook 2-3 minutes until thick and glossy with a dark amber sheen.

Return the chicken to the pan and turn to coat in the teriyaki sauce until the glaze clings to the thighs and looks sticky and shiny.

Serve the glazed chicken over steamed rice and drizzle with any extra sauce from the pan for a glossy finish. Garnish with sesame seeds and sliced green onions so the coating looks embedded and slightly speckled.