1. Peel and cut potatoes into quarters lengthwise. Remove any spots, lumps or knots.
2. Place potatoes in a large pot (6-quart) and cover with cold, salted water by an inch. Bring to a boil, reduce heat down to medium-low and allow to simmer until fork tender (about 15-20minutes).
3. When potatoes are just about done (nearly fork-tender), heat.1 tablespoon of the butter in a small pan over medium-high heat. Sauté garlic until fragrant, about 1 minute. Set aside and keep warm.
4. Drain cooked potatoes, add in the remaining butter, cooked garlic, hot milk and sour cream.
5. Mash the potatoes with a potato masher until smooth, adding little extra milk if needed.
6. Add in the parmesan cheese. Season with salt and pepper to taste.