Place the peeled and chunked potatoes and the garlic cloves in a large pot. Cover with cold water by about 1 inch and season the cooking water with salt.
Bring the water to a boil over high heat, then reduce the heat to medium and simmer gently for 15 to 20 minutes, or until the potatoes are fork-tender.
Drain the potatoes and garlic in a colander, then return them to the empty pot.
Begin mashing the potatoes and garlic with a potato masher until they are partially broken down.
Add 1/2 cup (1 stick) unsalted butter, 1 cup whole milk, 1½ teaspoons sea salt, and freshly ground black pepper (several grinds). Continue mashing until the potatoes reach your desired consistency.
If using, fold in 1/4 to 1/2 cup sour cream with a spatula or wooden spoon until evenly incorporated.
Taste and adjust seasoning as needed. Serve topped with additional butter and chopped fresh chives.