Place the duck on a cutting board and pat it very dry with paper towels, then sprinkle kosher salt all over, rubbing it in with your hands.

Set a roasting rack over a roasting pan, lay the duck on the rack, and chill it uncovered in the fridge for about 8 hours so the skin dries out.

When ready to cook, add boiling water, honey, soy sauce, and rice vinegar to a small saucepan and stir with a spoon over low heat just until smooth, then turn off the heat.

Set the duck on the counter, and use the spoon to brush the warm glaze all over the skin while it is still on the rack over the roasting pan.

Heat the oven to 400°F, then slide the roasting pan and rack with the duck into the center of the oven and roast about 1 hour 20 minutes until the skin is deep golden and very crisp when tapped with tongs.

Transfer the duck to a cutting board with tongs and let it rest 10 minutes, then use a sharp knife to slice off strips of skin and meat.

Warm the pancakes, then on a cutting board or plate spread a little hoisin sauce with a spoon, add duck slices, green onion, and cucumber, roll up, and serve right away.