Pancetta, an Italian bacon cured with salt and spices, adds depth of flavor to this chunky seafood soup. Be careful when seasoning, because pancetta, clams, and clam juice tend to be salty enough. Look for chowder clams or ocean quahogs, which are just the right size and flavor for this recipe. Purchase shells that are tightly closed; otherwise, tap them lightly, and if they don't snap shut discard them immediately. You can make broth a day in advance and store, covered, in refrigerator; add clams just before serving, and cook until clams open.