In a small bowl, whisk together soy sauce, dark brown sugar, water, Chinese five spice, garlic powder, red food coloring (if using), kosher salt, and white pepper to make the char siu marinade.
Reserve half the marinade in a small airtight container for basting later.
Trim excess fat and skin from chicken thighs, then flatten by partially cutting the meat from the bone, leaving about one-third attached.
Place chicken thighs in a large airtight container or resealable bag. Pour remaining marinade over chicken, toss to coat well, and refrigerate for at least 12 hours, up to 3 days.
About 30 minutes before roasting, remove chicken from refrigerator to come to room temperature. Preheat oven to 425°F. Position one rack center and another under the broiler.
Line a rimmed baking sheet with parchment or foil and lightly spray a wire rack to fit inside the sheet.
Place the marinated chicken thighs on the wire rack, discarding excess marinade.
Roast chicken on the center rack for 30 minutes, basting with reserved marinade and rotating the pan every 15 minutes, until chicken reaches 160°F and begins to caramelize.
Optional: Preheat broiler to high. Broil chicken 3 to 4 minutes, turning halfway through, until skin is charred and caramelized. Watch closely to avoid burning.
Let chicken rest a few minutes before slicing. Serve over rice or noodles, or use in steamed buns, fried rice, or stir-fries.