Stir 2 tablespoons gochujang, 2 tablespoons honey, 1 tablespoon toasted sesame oil, 1 tablespoon rice vinegar, and 1 teaspoon kosher salt together in an 8x8-inch baking dish (broiler-safe if you plan to broil the chicken at the end).
Pat 4 bone-in, skin-on chicken thighs dry with paper towels. Use your fingers to gently loosen and pull the skin away from the meat without removing it. Place in the baking dish and rub all over with the gochujang mixture, working it gently under the skin. Arrange skin-side up in a single layer. Let marinate at room temperature while the oven heats, at least 20 minutes. Meanwhile, heat the oven to 425ºF.
Roast until the chicken is lightly browned, cooked through, and an instant-read thermometer inserted into the thickest part registers at least 165°F, 25 to 35 minutes. For crispy skin, set the oven to broil. Broil until the skin is browned and crisp, 2 to 3 minutes. Garnish with thinly sliced scallions if desired.
