Pat dry the chicken dry with paper towels to make it easier to move and spatchcock. Flip the chicken over so it is breast-side down. Use kitchen shears to cut out the back bone: Starting from the tail-end of the chicken, cut along one side of the back bone, then repeat on the other side to remove the backbone. Keep the cuts as close to the spine as possible — this may require a little more pressure above the thigh bone. Flip the chicken so it is now breast-side up. Use the heel of your hands to press down on the breast bone to flatten the chicken. You may hear a crack as you press. Season the chicken inside and out with kosher salt.
Heat a gas or charcoal for two-zone cooking. For a gas grill this may look like turning one or two burners to high and the others to medium low. For a charcoal grill, light the coals in a chimney starter and pour the hot coals into one side of the grill.
Make the Korean BBQ sauce by whisking together the soy sauce, gochujang, honey, sesame oil, ginger, garlic, and green onion in a small bowl.
Grill the chicken breast side down over high heat for 10 minutes, keeping an eye out for flare ups. The skin should get crispy and browned in some areas.
Use two sets of tongs to flip the chicken over onto the cooler area of the grill and adjust the heat to maintain a lower temperature (roughly 350 degrees F). Cook for 25 minutes. When the chicken is golden brown and about 140 degrees in the thickest part of the breast. Begin basting with the sauce every 5 minutes. The sauce should thicken and create a glaze on the chicken’s skin. Cook for 10 to 15 additional minutes until the chicken reaches 165 degrees F.
Rest the chicken for 10 minutes. Serve over rice with sesame seeds and green onions, if desired.