Step 1

To make the sauce: In medium bowl, whisk together chicken stock, soy sauce, rice syrup, gochujang, gochugaru, and sesame oil. Set aside.

Step 2

In medium saucepan, heat vegetable oil over medium-high heat until shimmering. Add in garlic and ginger and cook, stirring constantly until fragrant, but not browned, about 1 minute. Add in chilies and black pepper and continue to cook, stirring constantly, until fragrant, about 30 seconds. Stir in gochujang mixture and Sprite. Bring to a rapid simmer and continue to cook, stirring and scraping down sides of saucepan occasionally, until reduced by about half and sauce coats the back of a spoon, 6 to 8 minutes. Remove from heat and let cool to room temperature.

Step 3

Pat chicken thighs dry with paper towels and place skin-side down on cutting board. Working with one thigh at a time, run knife down either side of bone without cutting all the way through the thigh flesh and skin. Run knife under bone to detach it from the meat. Repeat with remaining chicken thighs. Transfer chicken to a large Ziploc bag and pour in half of the cooled sauce. Seal bag, removing as much air as possible, and toss to distribute sauce evenly. Place bag in refrigerator for at least 45 minutes, up to overnight. Transfer remaining sauce to an airtight container and store in refrigerator until ready to use.

Step 4

Light one chimney full of charcoal. When all the charcoal is lit and covered with gray ash, pour out and arrange the coals on one side of the charcoal grate. Set cooking grate in place, cover grill, and allow to preheat for 5 minutes. Clean and oil grilling grate. Place bones and chicken, skin side down, over hot side of grill and cook, flipping occasionally, until well charred all over, about 10 minutes total. Move bones and chicken to cool side of grill and, using poultry shears, cut chicken into roughly 1 1/2-inch pieces. Brush chicken all over with reserved sauce and move back over to hot side of the grill and cook, turning occasionally, until well charred, about 2 minutes more. Transfer chicken and bones to a 10-inch cast iron skillet.

Step 5

Preheat broiler. Sprinkle shredded mozzarella evenly over chicken pieces. Place skillet under broiler and cook until cheese is completely melted and bubbly, 1 to 2 minutes. Remove from oven and sprinkle with scallions and sesame seeds. Serve immediately.