Rub the spice mixture evenly over the two racks of ribs, on all sides. Cover with plastic wrap and refrigerate, at least 6 hours and up to overnight.
Preheat the oven to 300°F, and arrange an oven rack in the middle position. Using damp paper towels, wipe off most of the spice rub from the two racks of ribs. Slice the racks into single ribs by slicing between the rib bones. Arrange the ribs evenly within a large roasting pan. (It is okay if the do not fit in a single layer and you have to place some on top of each other.) Cover the roasting pan tightly with foil.
Bake for 1 hour at 300°F, then decrease the heat to 250°F and bake 1 hour longer. Decrease the heat again to 200°F and bake 1 hour longer, or until the meat is very tender and pulling away from the rib bones. Remove the ribs from the oven, and cool at room temperature, about 1 hour.
At this point, you can either refrigerate them overnight, covered in plastic wrap, or proceed with the the next step, once they are cool. (If you do refrigerate them, make sure you let them set at room temperature for about 1 hour before proceeding.)
Heat the grill on high until very hot, about 30 minutes. Here’s a way to tell if your grill is at the right temperature. Hold your hand right over it. If you you can keep it there for five seconds, it’s not hot enough. If you can’t hold it there for even half a second, it's too hot. When the grill is hot, soak a clean rag or a bunch of paper towels with a few tablespoons of vegetable oil, and using tongs, rub it over the grill rack.
Pour about 1 1/2 cups barbecue sauce into a medium-sized container or bowl. Dip the ribs, one at a time, into the barbecue sauce, shaking them gently to remove any excess sauce. Arrange the sauced-dipped ribs directly over the flame or hot coals, working with only as many as you feel comfortable handling at once. Grill, meat side down, about 1 minute, then flip the ribs over and grill, bone side down, about 1 minute more. You just want to give the ribs some nice grill marks and heat them up a bit. Remove the grilled ribs to a serving platter and serve immediately, with extra barbecue sauce on the side.
Barbecue Sauce
Transfer this mixture to a blender and puree until smooth, about 30 seconds. Transfer back to the saucepot. Stir in the rest of the ingredients (ketchup, tomato sauce, vinegar, Worcestershire sauce, lemon juice, Tabasco sauce, liquid smoke, molasses, brown sugar, Dijon mustard, dry mustard and black pepper) and stir to combine. Bring to a boil over medium-high heat, then reduce the heat to low and simmer, uncovered and stirring frequently, until thickened to the desired consistency, about 15 minutes.
