If you are using fresh clams for clam meats and juice, you'll need about 44~53 oz of fresh clams, depending on the variety you're using. To cook them, combine the clams in a pot with 1/4 cup sake wine (if you add the sake now, don't add it again later), then put the lid on and cook over medium-high heat for 2~3 min, until they're all opened. Remove the meats and discard the shells, then strain the clam juice through a fine sieve (you should have a bit more than 1 cup of liquid). Set aside.
In a large pot, cook olive oil and diced pancetta over medium heat until the fats are rendered and the pancetta is browned. Add diced onion, celery and bay leaves, then cook for another 5 min until softened and translucent. Add all-purpose flour and cook for another min, then add the whole milk, clam juice, sake wine (or not, if you have used it to cook the clams already), gochujang/Korean chili paste, miso paste, 1/2 of the diced potatoes, ground black and white pepper. Stir constantly until the mixture has come to a simmer, and that the gochujang and miso have completely dissolved. Put the lid on and turn the heat down to low, and simmer for 20 minute.
Meanwhile, combine the other 1/2 of the diced potatoes with heavy cream in a small pot. Put the lid on and cook over medium~medium-low heat for 5 min, until the potatoes are soft (careful not to burn the cream). Set aside.
After simmering, the soup-mixture will look horrifyingly broken (the fats and water have separated, and looks grainy). DO NOT PANIC. All is expected here. Transfer the mixture to a blender (or with a hand-held immersion blender), and blend until it's smooth and creamy again. Transfer back to the pot, then add the potatoes/cream and 25 fresh little neck clams. Simmer for 5 min on low heat (and I mean SIMMER! NEVER BOIL! or the mixture may break again), stir occasionally, until the clams have opened. Add the clam meats, diced chives and grated ginger, taste, and re-season with fish sauce if needed.
Add 1 tsp of unsalted butter (2 if you want -- ) on top of each serving, and a little dust of ground white pepper and chili flakes (optional). Serve with rustic country bread or hot steamed rice.