Place the peeled and chopped potatoes in a large pot and cover with cold water by about 1 inch. Add 1 teaspoon of salt.

Bring to a boil over high heat, then reduce to a simmer. Cook until the potatoes are very tender when pierced with a fork, about 12–15 minutes.

While the potatoes cook, melt 2 tablespoons of butter in a small skillet over medium heat. Add the smashed garlic cloves and cook, stirring, until fragrant and lightly golden, about 2–3 minutes. Remove garlic and let cool slightly. (Alternatively, roast the garlic for a milder, sweeter flavor.)

Reserve about 1/2 cup of the starchy cooking water, then drain the potatoes well.

Return the drained potatoes to the pot over low heat for 1 minute to evaporate excess moisture, shaking the pot occasionally.

Use a masher or ricer to mash the potatoes to your desired texture.

In a small saucepan or in the microwave, warm the milk (or cream) with the remaining 2 tablespoons of butter until the butter is melted and the mixture is warm but not boiling.

Stir the warm milk-butter mixture into the mashed potatoes. Add the cooked garlic (minced), grated cheddar, sour cream if using, and about 2–3 tablespoons of the reserved cooking water as needed to reach a creamy consistency.

Season with black pepper and additional salt to taste. Fold gently until everything is combined and the cheese is melted.

Transfer to a serving bowl, top with a pat of butter and a sprinkle of chopped chives or parsley. Serve hot.