Prepare the grill for indirect cooking over low heat ( 120° to 180°C).
Combine the rub ingredients. Using a dull dinner knife, slide the tip under the membrane covering the back of each rack of ribs. Lift and loosen the membrane until you can pry it up, then grab a corner of it with a paper towel and pull it off. Season the racks on both sides with the rub. Wrap each rack in a sheet of heavy-duty aluminum foil, crimping the seams well and taking care not to puncture the foil. Grill the foil-wrapped racks, bone side down, over indirect low heat, with the lid closed, for 2 hours.
Combine the rub ingredients. Using a dull dinner knife, slide the tip under the membrane covering the back of each rack of ribs. Lift and loosen the membrane until you can pry it up, then grab a corner of it with a paper towel and pull it off. Season the racks on both sides with the rub. Wrap each rack in a sheet of heavy-duty aluminum foil, crimping the seams well and taking care not to puncture the foil. Grill the foil-wrapped racks, bone side down, over low heat, with the lid closed, for 2 hours.
Meanwhile, in a medium saucepan combine the sauce ingredients. Bring to a boil over high heat on the stove, stirring to dissolve the sugar, and then reduce the heat to medium-high and cook at a hard simmer until the sauce is reduced to 120 milliliters, 30 to 35 minutes (or longer), stirring occasionally. Reserve 3 tablespoons sauce to use later for a dipping sauce. Don’t wash the saucepan.
Remove the racks from the grill and transfer to a sheet pan. Carefully unwrap one end of each foil packet, tipping and pouring the collected drippings into a glass measuring cup or heatproof bowl; reserve. Unwrap the racks and discard the foil. Season the racks lightly with salt.
Return the racks to the grill, meaty side down, and brush some of the remaining sauce on the bone side of each rack. Cook over indirect low heat, with the lid closed, for 15 minutes. Use a brush to spread out any of the sauce that has accumulated in the curve of the racks. Turn the ribs over and brush some of the sauce onto the meaty side. Close the lid and continue grilling over indirect low heat for 30 to 45 minutes more, brushing the racks with sauce every 15 minutes. The ribs are done when the meat has shrunk back from most of the bones by 6 millimeters or more. When you lift a rack by picking up one end with tongs, the rack should bend in the middle and the meat should tear easily. Season with the sesame seeds.
Return the racks to the grill, meaty side down, and brush some of the remaining sauce on the bone side of each rack. Cook over low heat, with the lid closed, for 15 minutes. Use a brush to spread out any of the sauce that has accumulated in the curve of the racks. Turn the ribs over and brush some of the sauce onto the meaty side. Close the lid and continue grilling over low heat for 30 to 45 minutes more, brushing the racks with sauce every 15 minutes. The ribs are done when the meat has shrunk back from most of the bones by 6 millimeters or more. When you lift a rack by picking up one end with tongs, the rack should bend in the middle and the meat should tear easily. Season with the sesame seeds.
In the saucepan you used for the sauce, combine the reserved drippings, the reserved 3 tablespoons sauce, and the soy sauce. Bring to a boil. Stir in the dissolved cornstarch and simmer until the sauce is slightly thickened and translucent, about 1 minute, whisking constantly. Remove from the heat.
Remove the racks from the grill and let rest for 5 to 10 minutes. Reheat the dipping sauce, if necessary. Cut the racks into individual ribs and serve warm with the sauce.
