Cutting or peeling potatoes before boiling is begging for waterlogged potatoes that get mealy when hot and gummy when cold. Moral of the story: never, ever boil a potato that isn’t intact. Nevah! Evah! The skin is porous enough to allow moisture in to hydrate the flesh of the potato while protecting against overhydration, and that barrier is a necessity.
Place the potatoes (whole and unpeeled) into a large pot and cover with cold water. Bring the water to a boil, add a generous pinch of salt (preferably large grain kosher or sea salt) and reduce the heat so that the potatoes are simmering. Cook at a simmer until the potatoes are fork tender, for six good sized potatoes that will be about 20 minutes.
About fifteen minutes into the potatoes cooking process, put milk into a small saucepan over low-medium heat. Don’t let it boil, but let it get nice and warm.
When the potatoes are tender, drain off the water and return the potatoes, in the original cooking pot, back to the stove. Let the cooked potatoes sit on low heat for 2-3 minutes to remove any excess water lurking around.
Run the dry cooked potatoes through a potato ricer into a large mixing bowl. (Add roasted garlic and any other herbs/spices at this time except salt. Save salt for last.) Add the butter to the potatoes and mix it through so it has time to melt well. Then add the hot milk, in small batches, and stir everything until smooth. Add salt to taste, and serve right away.