Whisk together all marinade ingredients in a bowl until sugar dissolves. Add chicken pieces and toss to coat. Cover and refrigerate for 12–24 hours, turning once or twice. (Remove chicken and reserve the marinade liquid completely.

Heat oil in a large, heavy skillet (cast iron or stainless) over medium-high until shimmering. Add chicken in a single layer without crowding, work in batches if needed. Sear 3-4 minutes per side until browned and cooked through to an internal temperature of 165°F.

While the chicken cooks, pour the reserved marinade into a small saucepan. Bring to a boil, reduce to a simmer, and cook 5 minutes. Whisk in the cornstarch slurry. Continue stirring until the sauce turns glossy and thickens enough to coat a spoon, about 2 minutes more.

Remove the saucepan from the heat. Add the cold butter and swirl or whisk until fully incorporated and glossy. Do not return to the heat after adding butter; it will break. Pour sauce over cooked chicken in the skillet. Toss to coat. Serve immediately over steamed white rice.