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 4 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 2 minutes. Flip the chicken and brush on the reserved marinade. Flip the chicken about every 7 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 5 minutes, cut into serving pieces, and serve.
Reprinted with permission from The Japanese Grill: From Classic Yakitori to Steak, Seafood, and Vegetables by Tadashi Ono & Harris Salat, copyright (c) 2011. Published by Ten Speed Press, a division of Random House, Inc.