Preheat oven to 425°F; wash and dry the potatoes; poke holes in each potato with a fork; place directly on middle rack; bake one hour.
When cooled, cut each potato in half lengthwise; gently remove the flesh of each half with a spoon, to within a quarter inch of the skin; place the removed potato flesh in a mixing bowl.
Add butter; use a potato masher OR a mixer until butter is mixed in and potatoes are lumpy but mashed.
Add milk, sour cream, garlic, salt, and pepper; mix vigorously until well combined and a bit “fluffy”.
Spoon (or pipe) mashed potatoes into each potato skin half until mounded.
Preheat oven to 425°F; place potato skin halves snuggly on a baking sheet lined with aluminum foil or lightly sprayed with non-stick cooking spray.
Bake ~15 minutes; remove and top each potato half with cheese; return to oven and bake until cheese is melted and beginning to brown (~3 minutes).
