Rinse the duck and pat it completely dry. Remove the neck and giblets and discard. Remove excess fat and discard. Prick the skin of the duck all over with a fork, particularly around the breast.
In a small bowl whisk together the baking soda, salt, pepper, 5-spice powder, soy sauce and brown sugar. The mixture will fizz. Rub the mixture all over the duck then place on a plate and refrigerate, uncovered, and let dry overnight.
The next day, use your fingers to gently separate the skin of the duck from the meat. Pat the duck completely dry with paper towels inside and out.
Bring a quart of water to a boil. Place a duck on a roasting rack over the sink. Blanch the duck with the boiling water by pouring it over it. The skin will tighten and dry out more.
Preheat the oven to 425ºF (220ºC). Place the duck on a rack in a shallow roasting pan and cover with aluminum foil. Roast for 30 minutes. Remove the foil and prick the duck again.
Continue roasting, uncovered, until the juices run slightly pink when pricked in the thickest part of the thigh (the internal temperature should read 135ºF/ 57ºC), about 40 to 45 minutes longer. Remove from the oven and let stand for 10 minutes.
Carve the duck and serve with the pancakes, hoisin, cucumber and green onions.