Preheat oven to 400 degrees. Season ribs with chili powder, salt, and pepper. Stack slabs on a double layer of aluminum foil; wrap tightly. Place on a rimmed baking sheet. Cook until meat is fork-tender, 1 1/2 to 2 hours.
Meanwhile, make barbecue sauce: In a medium saucepan, melt butter over medium heat. Add onion and garlic; season with salt and pepper. Cook until soft, 2 to 3 minutes. Add ketchup, vinegar, Worcestershire, cayenne, and desired variation (see ingredients above). Bring to a simmer. Cook, stirring occasionally, until slightly thick, about 5 minutes. Set aside.
Heat grill to medium-high; lightly oil grates. Carefully remove ribs from foil, pouring off any accumulated liquid. Brush ribs generously with sauce; grill until charred, 2 to 3 minutes per side. Serve with extra sauce, if desired.
The base of our three sauces is made with ketchup, vinegar, onion, garlic, and cayenne. Add one of the variations for a sweet, spicy, or smoky sauce.
Desired variations (Kansas City style: 1/2 cup packed light-brown sugar, 2 tablespoons molasses, and 1 tablespoon yellow mustard; Memphis style: 1/2 teaspoon hot sauce (such as Tabasco) and 1/4 cup sugar; Dallas style: 1 tablespoon chopped chipotle in adobo sauce and 1/2 teaspoon ground cumin)