Place the potatoes in a single layer in a large pot. Cover with at least 1-inch of water. Season with one tablespoon of salt. Place over medium heat and let simmer for 40-45 minutes or until potatoes are easily pierced with a knife. Once cooked, remove the potatoes from the water to an ice bath and let cool slightly. Discard the water from the pot and wipe clean with a kitchen towel and set back on the stove top.
Once the potatoes are cool enough to handle, use a paring knife to remove the skins from the potatoes. Put the potatoes through a food mill or potato ricer into a mixing bowl. Set aside.
Place the milk into the pot and bring to a simmer. Add the garlic and let it steep for a few minutes or until garlic is a fragrant and beginning to soften slightly. Discard the garlic and turn the heat to low. Using a whisk, whisk in the butter a few chunks at a time until the butter is well incorporated. Once the butter is melted, add in the riced potatoes and, using a wooden spoon or rubber spatula, stir to incorporate. Once smooth, season with two teaspoons of salt and white pepper.
Pour the potatoes into a serving dish and serve hot. If serving later, pour into a casserole dish, cover with foil and keep warm in a 250°F oven until ready to serve.