1. In a heavy 4- to 6-quart saucepan or soup kettle, bring 2 quarts of chicken stock to a bubbling boil. Add the beans and boil uncovered for 2 minutes. Remove the pan from the heat and let the beans soak for 1 hour.
2. Meanwhile, melt 1 tablespoon of butter in a heavy 6 to 8-inch skillet. When the foam subsides, stir in the onion, carrot and leeks, and cook, stirring frequently, for 5 minutes, or until they are barely tender. Set the vegetables aside in the skillet. Blanch the salt pork, if you are using it, in 2 cups of water in a small saucepan by simmering it uncovered for to minutes. Set the pork aside on paper towels to drain.
3. Drain the soaked beans and measure the stock. Return the beans and stock to the saucepan together with enough additional stock or water to make 2 quarts. Add the vegetables, the blanched pork or ham bone, and the bouquet garni and bring to a boil over high heat. Reduce the heat and simmer the soup slowly, uncovered, for 2 hours, or until the beans are very tender. Discard the bouquet garni and the pork or ham bone.
4. Drain the beans, saving all the stock, and rub them through a sieve or food mill into a bowl. Return the pureed beans to the pan and add enough of the stock to make the soup as thick as heavy cream. If it is too thick, use more chicken stock to thin it. Taste and season the soup with salt and pepper. Bring the soup to a simmer over moderate heat and cook, stirring frequently, until it is hot.
5. Remove the pan from the heat and, off the heat, swirl in the 2 tablespoons of soft butter. To serve, ladle the soup into a large tureen or individual soup bowls and garnish with parsley.
