Step 1

Heat the oven to 300°F with a rack in the middle position. Line a rimmed baking sheet with extra-wide, heavy-duty foil, then set a wire rack in the baking sheet. In a medium bowl, whisk together the gochujang, vinegar, sugar, sesame oil, soy sauce, ginger and garlic. Measure out ¾ cup of the mixture, cover and refrigerate to use for glazing. Cut two 20-inch lengths of foil; set aside.

Step 2

Turn each rack of ribs meaty side down. Using a paring knife, cut a slit about 1 inch long in the membrane between the bones without cutting through meat. Lay one foil sheet on the countertop and set one rib rack on top. Coat the ribs on all sides with half of the remaining gochujang mixture, rubbing it into the meat and into the cuts in the membrane. Turn the ribs meaty side down on the foil. Draw the long sides of the foil together to cover the ribs and fold to seal tightly, then fold up and seal the short sides, creating a well-sealed packet. Repeat with the remaining foil sheet, rib rack and gochujang mixture. Place the packets seam side up on the prepared rack and bake until a skewer inserted into the meaty area between the bones meets no resistance, 2½ to 2¾ hours.

Step 3

Remove the ribs from the oven and let rest, still wrapped, for about 10 minutes. One packet at a time, carefully open one end of one of the foil and pour the liquid inside the packet into a 1-quart liquid measuring cup or medium bowl; you should have at least 2 cups. Unwrap the ribs and set them meaty side up directly on the rack; set aside while you prepare the glaze.

Step 4

Heat the broiler. Using a spoon, skim off and discard the fat from the liquid, then pour the liquid into a 12-inch skillet. Bring to a boil over medium-high, reduce to medium and cook, stirring occasionally, until the mixture is reduced to about 1 cup, about 15 minutes. Whisk in the reserved ¾ cup gochujang mixture, return to a simmer and cook, stirring occasionally, until thick enough that a spatula drawn through it leaves a trail, 5 to 7 minutes. Brush the surface and sides of the ribs with about half of the glaze.

Step 5

Broil the ribs until the glaze begins to char, about 2 minutes. Remove from the oven, brush on the remaining glaze, then continue to broil until lightly charred, another 3 to 5 minutes. Remove from the oven and sprinkle with the sesame seeds (if using). Let rest for about 15 minutes. Transfer the rib racks to a cutting board. Separate the ribs by cutting between the bones, then transfer to a platter.