Step 1 Scrub clams well and steam in a large pot with about 1 cup of water just until they open, 5 to 10 minutes, depending on their size. When cool enough to handle, remove clams from shells over cooking pot to catch juices. Pull black skin off the necks and, if clams are large, separate soft parts from the firm; chop firm parts. (If small, leave whole.) Strain broth through cheesecloth or dish towel and reserve.
Step 2 In a large kettle or soup pot, cook salt pork over medium heat until fat is rendered and pork bits are crispy, about 10 minutes. Remove with a slotted spoon to drain on paper towels and reserve, leaving drippings in pot.
Step 3Add onion to drippings and cook until it begins to soften, about 6 minutes. Add diced potatoes and clam liquor, bring to a boil, reduce heat to medium-low, and simmer, partly covered, until potatoes are almost tender, about 10 minutes. Add clams and simmer until potatoes are tender, about 5 minutes longer. Stir in milk and evaporated milk and heat through. Season with salt and pepper to taste. (Remove from heat and let chowder sit, at cool room temperature, at least 2 hours or refrigerate up to 2 days.)
Step 4Reheat over very low heat, stirring frequently, until chowder steams and is heated through. This chowder should not boil or it could curdle. Stir in butter until it melts and adjust seasonings if necessary. Ladle into shallow bowls to serve. Pass reserved salt pork bits for sprinkling on top if desired.