Place the diced potatoes in a large pot and fill with just enough water to cover about an inch above the tops of the potatoes. Bring to a boil over high heat for about 25 minutes or until the potatoes are soft and tender. Pierce with a fork to check tenderness.
Drain the water from the pot. Add butter, (finely minced) garlic, milk, sour cream, and mash with a potato masher. Stir in 1 cup of shredded cheddar cheese until combined and melted. Stir in salt and pepper to preferred taste. Sample the potatoes and add more milk or sour cream, to obtain the desired consistency. Add more salt and pepper as needed.