Step 1: Prepare the duck: The day before you plan to serve the duck, remove and discard the fat just inside the body cavities. Remove the package of giblets and set aside for another use. Rinse the duck, inside and out, under cold running water, then drain and blot dry, inside and out, with paper towels. Place the duck in a roasting pan and let stand, uncovered, in the refrigerator overnight to dry out the skin.
Step 2: Season the body cavity of the duck with salt, pepper, and 1/2 teaspoon of the five-spice powder. Place the garlic clove, scallion, and ginger slices in the body and neck cavities, then turn the duck over on its breast so that the back side is up. Using the tip of a sharp, slender knife, make a small slit in the fatty part of the duck under each wing and a slit on the underside of each thigh. Prick the duck skin all over with a sharp carving fork, being careful not to pierce the meat. Brush the outside of the duck all over with the sesame oil and rub the skin all over with more salt, pepper, and the remaining 1/2 teaspoon five-spice powder. Truss the duck with butcher’s string.
Step 3: Set the grill up for spit roasting and place a large drip pan under the spit. Preheat the grill to medium (about 300 degrees).
Step 4: Thread the duck onto the spit (it should run perpendicular to the spit). Spit-roast the duck until darkly browned and cooked well-done (which is how the Chinese eat their duck), or to an internal temperature of 180 degrees. Halfway through the cooking time, re-prick the skin to release the fat. The whole process will take 2 to 2 1/2 hours. From time to time, baste the duck with the fat that accumulates in the drip pan. If the skin starts to brown too much before the meat is cooked, lower the heat.
Step 5: Transfer the duck to a cutting board and let cool for a few minutes, then carve the skin off the bird. Cut it into thin slivers. Next, carve the meat off the carcass and cut it into thin slivers. Arrange these on a platter with the bowls of hoisin sauce, cucumber, and Pineapple Salsa. Have the steamed buns in a cloth-lined bread basket.
Step 6: Have each person spoon a little hoisin sauce in a steamed bun, then add duck skin, duck meat, cucumber, and Pineapple Salsa.