Fill a small baking pan two-thirds full with cold water and place on the lowest rung in the oven. This helps to keep the ribs moist during the lengthy cooking time.
Position a second rack in the middle of the oven. Preheat the oven to 275 degrees F. Line a large baking sheet with aluminum foil.
To prepare the ribs, in a small mixing bowl combine the salt, black pepper, light brown sugar, and cayenne, and toss to combine. Divide the spice mixture equally between the two racks of ribs and, with clean hands, rub into the meat well on both sides.
Place the ribs on the foil-lined baking sheet and bake for 2 hours, uncovered.
While the ribs are baking, make the BBQ sauce: In a medium nonreactive saucepan over medium-high heat, add the butter and, when melted, add the onion and cook until it is very soft, about 4 minutes. Add the garlic, paprika, Bayou Blast or other Creole seasoning, dry mustard, salt, crushed red pepper, black pepper, and cayenne, and cook for 2 minutes. Add the tomato paste and cook, stirring frequently, for 2 minutes, until the tomato paste begins to brown. Add the water, cider vinegar, and dark brown sugar and stir to combine. Reduce the heat to low and cook until the sauce has thickened and the flavors have come together, about 15 minutes. Set aside to cool.
When cooled, transfer the sauce to a blender. Puree until very smooth, 1 to 2 minutes, then transfer the sauce to a small nonreactive bowl and set aside.
When the ribs have baked for 2 hours, use oven mitts or pot holders to remove the baking sheet from the oven. Using a basting brush and tongs to turn the ribs, lightly coat both sides of both racks of ribs with some of the BBQ sauce. Return the ribs to the oven and bake for 15 minutes.
Using oven mitts or pot holders, remove the baking sheet from the oven and, using tongs, carefully turn the ribs. Return the ribs to the oven and bake for 15 minutes longer. Remove the ribs and let cool slightly.
Using a sharp knife, cut the racks of ribs into individual ribs for serving. Serve immediately, with the extra BBQ sauce at the table.
Yield: 2 racks of baby back pork ribs, serving 4 - 6; 3 cups of barbecue sauce