Peel potatoes and cut into rough chunks, about 1 to 2 inches cubed.
Transfer to a container of cold water to rinse. Change water 2 or 3 times until it runs clear.
Store overnight in refrigerator covered with water.
Cook:
Place potatoes in large pot and cover with cold water. Add 2 teaspoons of salt to the pot, cover and bring to a boil over high heat.
While potatoes cook, combine the buttermilk, cream, butter, garlic, and rosemary in a 2-quart saucepan and bring to a bare simmer over low heat, watching to make sure the mixture doesn't boil over. Keep the mixture barely bubbling until the potatoes are done.
When potatoes start to boil, ditch the lid, drop the heat to a simmer, and cook until EASILY crushed with tongs, 15 to 20 minutes.
Drain the potatoes, pop them back to the hot pot to dry, then add to stand mixer fitted with paddle attachment.
Strain Cream Mixture, then add to stand mixer. Blend until JUST smooth.