Make marinade and glaze. In a large bowl, add Gochujang, soy sauce, mirin, sugar, sesame oil, and garlic. Whisk until smooth. Scoop 1/3 cup of the marinade and set aside in a small bowl, for later glazing.
Score chicken. Make 2 or 3 cuts into the meaty side of each drumstick. Do not cut down to the bone, but about halfway through the meat. This will help the marinade to penetrate the meat.
Marinate chicken. Add scored drumsticks to the marinade, mixing until the chicken is fully coated.
Preheat oven. Turn on the oven to 350F/175C and let the drumsticks marinate while the oven preheats, about 15 minutes.
Bake chicken. Line a baking sheet with parchment paper or aluminum foil. Ad the drumsticks in an even layer, making sure not to crowd the pan. Bake until halfway cooked through, about 15 minutes.
Brush glaze. Add the reserved sauce from before, as a glaze. Brush chicken every 3-5 minutes until fully cooked, an additional 10-15 minutes. That's 25-30 minutes in total. This will ensure a deeper-flavored, glazed coating vs. roasting alone.
Broil. When the chicken is fully cooked, broil for the an additional 3-5 minutes to get those charred, crispy bits. Watch carefully so it doesn't burn!
Serve. Garnish with sesame seeds and green onion (optional). Serve with rice and steamed veggies. Enjoy!