Note: Yields 1/2 cup spice rub and about 2/3 cup sauce.
Make the spice rub: In a small bowl, stir together all the ingredients.
Cook the ribs: Position a rack in the center of the oven and heat the oven to 300°F Sprinkle and press 1/4 cup of the rub on both sides of each rib rack.
Put the racks, meaty side up, on a broiling pan or a wire roasting rack set over a baking sheet.
Lightly season the ribs with salt and put them in the oven.
After the first hour, rotate the pan every 30 minutes (If you use two baking sheets, switch their position in the oven, too.) The ribs will sizzle gently as they cook, and they’ll become tender after about 2 - 3 hours in the oven.
To test for doneness, pick up the center of the ribs with tongs; the ends of the ribs should flop downward (this means the fat and cartilage have broken down), and a skewer inserted between the ribs should meet little resistance.
If the meat between the ribs is still tough, keep cooking, checking every 15 minutes and rotating the pan.
Meanwhile, make the dipping sauce: In a medium saucepan over medium heat, bring the soy sauce, sugar, rice vinegar, ginger, and sesame oil to a simmer, stirring occasionally.
Remove from the heat and let cool to room temperature.
The sauce will keep for about a week in the refrigerator.
Cut and serve: When the ribs are done, remove them from the oven, put them on a cutting board meaty side down (so they’re easier to slice), and slice them into individual ribs.
Arrange on a platter and serve with the sauce on the side.
Or, stack the ribs, drizzle with some of the dipping sauce (serve the rest on the side), and garnish with the sliced scallions.