Place potatoes and smashed garlic clove into a large saucepan; cover with cold water and season generously with salt.
Bring to a gentle boil over medium-high heat, then reduce to a simmer; cook until fork-tender, about 15–20 minutes.
Drain thoroughly, discard the garlic, then return potatoes to the pan over low heat for 1–2 minutes to dry out excess moisture.
Stir in butter until it melts into a rich pool, pour in warm milk and sour cream, then mash to a creamy yet slightly textured consistency. Season with salt and pepper.
Notes
Testing Note: I found that reheating with a splash of milk brings back that just-made silkiness for leftovers.Food Safety: Cool mashed potatoes within two hours and refrigerate in an airtight container for up to 3 days.