Mix together the soy sauce, sake, ginger, garlic, tobanjan, brown sugar, and salt in a bowl to make the marinade. Pour three-fourths of the marinade into a baking dish or rimmed sheet pan; reserve the rest. Lay the chicken in the marinade and flip the halves four times to generously coat all over. Use a rubber spatula or your hands to make sure all the crevices are well coated. Marinate the chicken halves for 10 minutes, turning once.
Preheat a grill to medium. Grill the chicken, covered, for about 35 minutes. Start by grilling skin side down for about two minutes. Flip the chicken and brush on the reserved marinade.
Flip the chicken about every seven minutes, brushing on more marinade after each turn. The chickens are done when very juicy with meat pulling away from the bone. Rest the chicken for five minutes, cut into serving pieces, and serve.
