If using fresh clams separate the soft part of the clams (stomach) from the hard part surrounding it. Finely chop the hard part and set aside, and slice each soft part in two and reserve separately.
Strain the clam liquor through two layers of cheesecloth and set aside. If using canned clams, drain the clams in a fine sieve over a bowl and reserve the liquid.
In a large soup pot sauté the salt pork over low heat until crisp and they have rendered all their fat. Remove and reserve. Add the onions to the fat remaining in the pot and cook over moderate heat for about 5 minutes, until they are translucent but not brown. Stir in the reserved clam liquor, the finely chopped fresh clams (do not add canned clams at this point), the milk, and the potatoes. Cover and simmer 10 to 15 minutes, until the potatoes are tender.
Stir in the reserved soft parts of the clams (or the canned clams), the reserved salt pork or bacon, and the cream, and simmer for an additional 3 minutes. Add seasoning salt. Adjust the seasoning with salt and pepper.
Allow to rest off heat for one to two hours, then reheat immediately before serving. Ladle into warm bowls, and place a teaspoon of butter on top of each serving.