Clean and debeard the mussels. Stem them open. Strain the broth; you should have 4 cups of broth (and 1 pound of mussel meat). Cover the shelled mussels and keep refrigerated until ready to use.
Heat a 4- to 6-quart heavy pot over low heat and add the diced salt pork. Once it has rendered a few tablespoons of fat, increase the heat to medium and cook until the salt pork is a crisp golden brown. Using a slotted spoon, transfer the cracklings to a small ovenproof dish, leaving the fat in the pot, and reserve until later.
Add the butter, ginger, garlic, bell pepper, onion, curry powder, and cayenne pepper to the pot and saute, stirring occasionally with a wooden spoon, for about 10 minutes, until the pepper and onion are softened but not browned.
Add the white potatoes and the reserved mussel broth, turn up the heat, and bring to a boil. Cover and cook the potatoes vigorously for 5 minutes. Add the sweet potatoes. The broth should just barely cover the potatoes; if it doesn't, add enough water to cover. Lower the heat to medium, cover, and cook for 6 to 8 minutes longer, until the white and sweet potatoes are soft on the outside but still firm in the center. If the broth hasn't thickened lightly, smash a few potatoes and sweet potatoes against the side of the pot and cook a minute or two longer to release their starch.
Remove the pot from the heat and stir in the mussels and cream. Mussels are not as salty as clams, but you will still need to exercise a light hand as you season the chowder with salt. Add black pepper to taste. If you are not serving the chowder within the hour, let it cool a bit, then refrigerate; cover the chowder after it has chilled completely. Otherwise, let it sit at room temperature for up to an hour, allowing the flavors to meld.
When ready to serve, reheat the chowder over low heat; don't let it boil. Warm the cracklings in a low oven (200 degrees F) for a few minutes.
Ladle the chowder into cups or bowls, making sure that the mussels, bell pepper, onion, and potatoes are evenly divided. Scatter the cracklings over the individual servings and sprinkle with the chopped cilantro and scallions.
Courtesy of Jasper White's FIFTY CHOWDERS (Scribner, 2000)