Place the peeled and chunked potatoes and whole garlic cloves in a large pot. Cover with cold water, add a generous pinch of salt, and bring to a boil over high heat.
Reduce the heat to medium and simmer for 15-20 minutes, or until the potatoes are fork-tender. Drain the potatoes and garlic in a colander, then return them to the pot.
Add the softened butter, heavy cream, and sour cream (if using) to the pot with the potatoes and garlic. Mash everything together using a potato masher or hand mixer until smooth and creamy.
Season with salt and black pepper to taste. If the mixture is too thick, add more cream or milk, a tablespoon at a time, until you reach your desired consistency.
Transfer the mashed potatoes to a serving bowl. Garnish with chopped chives or parsley if desired.
Serve warm alongside your favorite main dish and enjoy your creamy garlic mashed potatoes!