4 tablespoons (1/2 stick) unsalted butter 1 large yellow onion, chopped 4 cloves garlic, chopped 1 cup ketchup 3 tablespoons tomato paste One 12-ounce can Dr. Pepper 1/2 cup cider vinegar 1/3 cup Worcestershire sauce 1/2 cup packed dark brown sugar 2 teaspoons ancho or New Mexican chili powder 1 teaspoon fine-ground white pepper 1 teaspoon kosher salt