Peel and rinse the Russet potatoes.
Cut the potatoes into large 1-inch chunks.
Put the cut potatoes in a pot with cold water and add the Kosher salt.
Bring the potatoes to a simmer, and reduce the heat to medium-low.
Boil the potatoes on medium-low heat for roughly 20-25 minutes, or until they are completely tender.
Drain the potatoes and pass them through a food mill, or mash them well with a potato masher.
While the potatoes are still hot, add the 12 Tbsp of softened butter, the crème fraiche, and the whole-grain mustard.
Use a rubber spatula to gently incorporate the whole-grain mustard, crème fraiche, and butter until you have a smooth and buttery mashed potato puree.
Gently fold in the fresh chives.
Taste, and season with more Kosher salt & freshly-ground black pepper, if desired.
Serve hot, alongside roasted or braised meats or with a hearty stew such as Boeuf Bourguignon.