Preheat smoker to 325° F or oven to 450° F.
To brine: Mix the brine ingredients in a large baggie. Season the drummers with salt and pepper and add to the brine. Refrigerate for 3 hours or overnight. After brined, bring to room temperature and pat dry. Spray evenly with cooking spray.
If using a smoker: Place a wire rack over a baking sheet and arrange the drummers on the rack. Cook for about 75 minutes, or until the skin is tight and the chicken is cooked. If using the oven: roast for 30-45 minutes or until crisp and cooked through.
For the sauce, melt the butter in a pan over medium heat. Add the garlic and ginger, and swirl for a minute. Then add the soy, mirin, rice vinegar, gochujang, brown sugar, and, espresso powder. Stir to combine. Drop the heat to low and simmer to thicken for about 10-15 minutes. If needed, add 1 Tablespoon of water at a time until desired consistency.
Toss the drummers with sauce and top with green onion and crispy hatch chiles.