Bring a large pot of water to a boil, generously salt. Add potato chunks and whole garlic cloves and simmer until fork tender, about 15 minutes.
Meanwhile, while potatoes cook, heat butter and whole milk in small sauce pan until butter is melted and mixture is hot but not boiling. Set aside.
When potatoes and garlic are tender, drain in colander. Place food mill fitted with medium disk on top of same pot and working in batches, pass potatoes and garlic through mill, turning the handle back and forth.
After finished mashing, add hot butter/whole milk mixture and stir well with rubber spatula. Gradually add buttermilk and continue to mix until desired creaminess, about ¾ cup of buttermilk, maybe more. Taste for salt, adding a teaspoon or two, depending on butter used, and some fresh cracked black pepper. Serve warm and enjoy!