Step 1

Adjust oven rack to middle position, place 12-inch cast-iron skillet on rack, and heat oven to 500 degrees. Meanwhile, pat chicken dry with paper towels and season with pepper.

Step 2

When oven reaches 500 degrees, remove skillet from oven using potholders and place over medium heat; turn off oven. Being careful of hot skillet handle, add oil and heat until just smoking. Place chicken skin side down in skillet. Place weighted Dutch oven on top of chicken and cook until skin is deep brown and very crispy, 16 to 20 minutes, checking browning after 10 minutes and adjusting heat as needed.

Step 3

Remove weighted pot and flip chicken. Continue to cook, without weight, until chicken is lightly browned on second side and registers 175 degrees, about 2 minutes; transfer to serving platter.

Step 4

Whisk soy sauce, sugar, mirin, garlic, ginger, and cornstarch together in bowl. Discard fat from skillet, then add soy sauce mixture and bring to simmer over medium heat. Cook, stirring occasionally, until sauce is thick and glossy, about 2 minutes. Stir in any accumulated chicken juices. Return chicken to skillet and turn to coat with sauce. Return chicken to serving platter, sprinkle with scallions, and serve, passing remaining sauce separately.