Place the peeled and cut Yukon Gold potatoes into a large pot and cover with cold water by about an inch.

If using garlic, add the smashed clove to the pot for a mild infusion. Bring the water to a boil over high heat, then reduce to a simmer and cook until the potatoes are very tender, about 15–20 minutes.

While the potatoes cook, heat the milk and butter together in a small saucepan over low heat until the butter is melted and the mixture is warm. Do not boil.

Drain the potatoes well and return them to the hot pot. Remove the garlic clove if used.

Mash the potatoes with a potato masher, ricer, or fork to your preferred texture. For extra-smooth results, use a ricer or food mill.

Gradually stir in the warm milk and butter mixture, a little at a time, until the potatoes reach the desired consistency.

Stir in the sour cream if using, then season with kosher salt and freshly ground black pepper. Taste and adjust seasoning.

Transfer to a serving bowl, top with a pat of butter and chopped chives or parsley if desired, and serve hot.