How to Make It
Place chicken, breast side down, on a cutting board. Using kitchen shears, cut along both sides of backbone; remove and discard backbone. Turn chicken breast side up; press firmly against breastbone with heel of your hand, splaying chicken open and flattening it slightly, butterfly style. Trim excess fat. Combine soy sauce and oil in a large ziplock plastic bag; add chicken. Seal bag; chill 1 to 2 hours. Drain chicken; discard marinade. Pat chicken dry.
Whisk together ketchup, corn syrup, vinegar, garlic, crushed red pepper, and 1/4 teaspoon salt until smooth. Set aside.
Preheat a gas grill to high (450°F to 550°F) on one side, or push hot coals to one side of a charcoal grill. Place chicken, breast side down, on oiled grates over lit side of grill. Grill chicken, uncovered, rotating (but not flipping) often, until skin is crispy, 6 to 8 minutes. (Make sure to keep rotating chicken to prevent flare-ups.) Place chicken, breast side up, over unlit side of grill, and grill, covered, until a thermometer inserted in thickest portion registers 165°F, about 35 minutes.
About 10 minutes before chicken is done, coat scallions and bell peppers with cooking spray. Grill, turning often, until lightly charred, 4 to 5 minutes for scallions and 6 to 7 minutes for bell peppers. Transfer vegetables to a cutting board. Cut scallions into 2-inch pieces, and cut bell peppers into strips.
Arrange grilled vegetables on a serving platter; sprinkle with remaining 1/4 teaspoon salt. Place chicken on top of vegetables, and brush chicken with 1/4 cup sauce. Sprinkle evenly with cilantro and sesame seeds, and serve with remaining sauce.