Step 1: Make the tare: Place the chicken stock, soy sauce, mirin, sugar, scallion white, garlic, ginger, and lemon zest in a nonreactive saucepan and bring to a boil over medium heat. Let the tare simmer until thick and syrupy, 6 to 10 minutes, stirring often to prevent scorching. Strain the tare into a deep narrow saucepan you can place on the grill; there must be at least 3 1/2 inches of tare in the pot. Discard the solids from the tare. Step 2: Make the yakitori: Cut the chicken into 1/2-inch cubes and thread it onto skewers. Fill each skewer only halfway; leave the other half bare as a handle. Typically, when skewering the meat, a Japanese grill master will intersperse lean pieces of chicken with pieces of fat or skin. You can grill only one type of chicken meat or you can serve a variety of chicken pieces: dark meat, white meat, chicken skin, and so on, skewering each part of the chicken separately. If you are not grilling the yakitori immediately, refrigerate them covered with plastic wrap. The yakitori can be prepared several hours ahead to this stage. Step 3: Set up the grill for direct grilling and preheat it to high. Step 4: When ready to cook, brush and grease the grill grate (a piece of chicken fat or skin held in tongs works great for greasing). Keep the pot of warm tare on one corner of the grill. If you are using a hibachi or other slender grill, arrange the chicken skewers on the hot grate so that the bare ends hang over the edge. On a larger grill, arrange the yakitori on the grate with an aluminum foil shield under the exposed ends of the skewers to keep them from burning. Step 5: Grill the yakitori until the chicken is partially cooked (it will be white on the outside), about 2 minutes per side. Dip each yakitori in the sauce, then return it to the grate. Continue grilling the yakitori until the chicken is well browned and cooked through, 1 to 2 minutes per side longer. The outside should cook to a shiny glaze and the meat should feel firm to the touch when done. Take care that the yakitori don’t burn. Step 6: Dip each grilled yakitori in the tare one more time and transfer them to a platter or plate. Sprinkle with sesame seeds and chopped scallion greens, if using, on top and serve at once. Although it’s not strictly traditional, I like to serve a little reserved tare in a tiny bowl as a dipping sauce.