Place the beans in a large pot or soup kettle. Pour in 8 cups of the stock and bring to a boil over high heat. Reduce the heat to medium and add the bay leaf and a pinch of salt. Cook, partially covered, stirring often and adjusting the heat to keep it at a slow steady simmer, until the beans are tender, about 2 hours. Remove and discard the bay leaf.
Soak the saffron in the remaining 1/2 cup chicken stock.
Heat the oil in a large skillet over medium-high heat. Add the chorizo and cook, stirring often, until lightly browned, about 5 minutes. Use a slotted spoon to transfer to a plate. Add the onion; reduce the heat to medium and cook, stirring often, until softened, about 5 minutes. Add the garlic and cook 1 minute longer. Stir in the red pepper and the paprika. Cook for 2 minutes longer, then transfer the contents of the skillet to the bean pot. Stir in the saffron with the soaking liquid, the chorizo, and the kale. Bring back to a simmer and cook just until the kale is wilted, about 5 minutes. Season with salt and pepper, stir in the vinegar, and serve hot in warmed soup bowls.
Sara Moulton Cooks at Home, Broadway Books, 2002