In a small bowl, combine the soy sauce, dark brown sugar, water, Chinese five spice, garlic powder, & red food coloring (if using). Season with 1 heaping teaspoon kosher salt & 1/4 teaspoon ground white pepper. Whisk to combine well. Transfer 1/2 of the marinade to a small airtight container or jar & reserve for basting the roasted chicken.
Trim excess skin & fat from the chicken thighs, then flatten them for faster, even cooking by partially cutting the meat away from the bone on both sides, leaving about 1/3 of the bone still attached. Place chicken in a large airtight container or resealable plastic bag & pour the remaining 1/2 of the char siu marinade over top. Toss to combine, coating the chicken in the marinade well. Cover/seal & transfer to the refrigerator to marinate for at least 12 hours or up to 3 days.
About 30 minutes before you’re ready to roast, remove chicken from refrigerator. Preheat oven to 425 degrees F, ensuring one oven rack is positioned in the center of the oven & a second oven rack is positioned directly under the broiler. Line a rimmed baking sheet with parchment or foil for easy clean up. Lightly spray a wire rack with cooking spray & set it in the baking sheet. Transfer the char siu marinated chicken thighs to a wire rack, shaking off & discarding any excess marinade.
Place the char siu chicken on the center rack in the preheated oven. Roast 30 minutes, basting the chicken & rotating the sheet pan every 15 minutes. To baste, use a pastry brush or silicone basting brush to generously brush the reserved char siu sauce all over the surface of the chicken. The char siu chicken is ready once it begins to caramelize around the edges & reaches an internal temperature of 160 degrees F, about 30 minutes.
Preheat your oven’s broiler to its highest setting. Place the roasted char siu chicken under the broiler. Broil 3-4 minutes, rotating the pan halfway through, until the surface of the char siu chicken is charred & caramelized as desired – be sure to keep a close eye on it as it broils, as things quickly transition from perfectly caramelized to burnt under the broiler!