Place the potato chunks and optional peeled garlic in a large saucepan. Cover with cold water by 1 inch and season the water lightly with salt.
Bring to a boil, then reduce to a steady simmer. Cook 15 to 18 minutes, until a knife slips easily into the largest piece.
Drain thoroughly and return the potatoes to the warm pot. Set over low heat for 30 seconds, shaking gently, to let excess moisture evaporate.
Mash the potatoes and garlic to your preferred texture.
Warm the milk and butter together until the butter melts. Add gradually, folding between additions, until the potatoes are creamy but still hold their shape.
Season with salt and pepper to taste.