1. Make the yakitori sauce: Arrange an oven rack 4″ from broiler and heat broiler to high. Remove bones from chicken legs and cut bones into 1″ pieces. Cut chicken meat into ¼″-thick slices; refrigerate until ready to use. Transfer chicken bones to a foil-lined baking sheet; broil, turning, until browned all over, about 10 minutes. Transfer bones to a 4-qt. saucepan and add mirin, soy sauce, sake, sugar, pepper, green parts of scallions, garlic, ginger, and 1 cup water; bring to a boil, and then reduce heat to medium-low. Cook until liquid is reduced by half, about 1½ hours. Pour through a fine strainer into a bowl; let yakitori sauce cool.