Step 1

Mix five-spice powder, salt, MSG, 1 Tbsp. white pepper, and 1 Tbsp. brown sugar in a small bowl.

Step 2

Place chicken, breast side down, on a cutting board and, using kitchen shears or a large knife, cut along both sides of backbone to remove (you can ask a butcher to do this for you if you prefer); discard (or save for making stock). Open up chicken and turn breast side up. Press down on center of breast to flatten chicken—you should hear the breastbone crack. Tuck wings behind breast, then tuck in legs so bottoms of drumsticks are pointed away from body and chicken is as flat as possible. Sprinkle spice mixture all over chicken. Let sit at room temperature 20 minutes, or cover and chill up to 1 day. If chilling, let sit at room temperature 1 hour before grilling.

Step 3

Prepare a grill for medium-high indirect heat (for a charcoal grill, bank coals on one side of grill; for a gas grill, leave one or two burners off). Lightly oil grate. Grill chicken, breast side up, over indirect heat, undisturbed, until browned and lightly charred underneath, about 20 minutes.

Step 4

Turn chicken breast side down and move over direct heat. Grill, turning often and returning to indirect heat as needed, until well-browned, cooked through, and an instant-read thermometer inserted into the thickest part of a thigh registers 165° and into thickest part of breast registers 160°, 15–20 minutes. Transfer chicken to a cutting board and let rest 5 minutes.

Step 5

Brush scallions with remaining 1 Tbsp. vegetable oil and grill, turning occasionally, until charred in spots, 6–8 minutes. Transfer to a cutting board and thinly slice.

Step 6

Mix scallions, garlic, chiles, vinegar, soy sauce, sesame oil, remaining ½ tsp. white pepper, and remaining ½ tsp. brown sugar in a small bowl.

Step 7

Carve chicken and arrange on a platter. Serve with charred scallion sauce for spooning over.