Place the hoisin sauce, sugar, and five spice powder in a nonreactive mixing bowl and whisk to mix. Add the soy sauce, rice wine, and sesame oil and whisk until the sugar dissolves. Stir in the garlic, ginger, and scallion whites. Set one third of the marinade aside to make a sauce.

Prepare the ribs: Place a rack of ribs meat side down on a work surface. Remove the thin, papery membrane from the back of the rack by inserting a slender implement, such as a butter knife or the tip of a meat thermometer, under it. The best place to start is on one of the middle bones. Using a dishcloth, paper towel, or pliers to gain a secure grip, peel off the membrane. Repeat with the remaining rack.

Place the ribs in a nonreactive roasting pan or baking dish just large enough to hold them. Pour the remaining marinade over the ribs and spread it all over the racks with a rubber spatula, turning to coat both sides. Let the ribs marinate, covered, in the refrigerator for at least 4 hours or as long as overnight, turning them 3 or 4 times. The longer the ribs marinate, the richer the flavor will be. (The ribs can also be marinated in large heavy resealable plastic bags.)

Set up the grill for indirect grilling and preheat to medium. Place a large drip pan in the center of the grill under the grate.

When ready to cook, brush and oil the grill grate. Drain the ribs well and place them in the center of the grate bone side down over the drip pan and away from the heat. (If your grill has limited space, stand the racks of ribs upright in a rib rack.) If cooking on a charcoal grill, toss half of the wood chips on each mound of coals. Cover the grill and cook the ribs until dark brown and very crisp on the outside and tender enough to pull apart with your fingers, 1 1/4 to 1 1/2 hours. When ribs are done, the meat will have shrunk back from the ends of the bones by about 1/4 inch. If using a charcoal grill, replenish the coals as needed.

Meanwhile, transfer the reserved marinade to a nonreactive saucepan, let come to a gentle simmer over medium heat, and cook until thick and flavorful, about 3 minutes. Let the resulting sauce cool to room temperature, then strain it into an attractive serving bowl.

Transfer the ribs to a large platter or cutting board. Let the ribs rest for a few minutes, then cut the racks in half or into individual ribs. Brush or drizzle the ribs with some of the sauce and sprinkle the scallion greens on top. Serve at once with the remaining sauce on the side.