Wash and scrub the red potatoes, then dice into roughly 1-inch cubes.
Place the potatoes in a large heavy-bottomed pot and cover with cold water by about 1 inch. Stir in 2 teaspoons kosher salt.
Bring to a rapid boil over high heat, then reduce to medium and simmer until the potatoes are very fork-tender, about 20–25 minutes.
Drain the potatoes and return them to the hot pan. Place the pan over low heat for about 1 minute to evaporate excess moisture.
Using a potato masher, mash the potatoes until mostly smooth with some texture remaining as desired.
Add 1 teaspoon kosher salt, 1 teaspoon garlic powder, 3 tablespoons melted butter, and 1/4 cup warmed milk. Stir until creamy.
Stir in 1/2 cup sour cream. If needed, add additional melted butter and warmed milk (up to an extra 1/4 cup) to reach your preferred consistency and richness. Taste and adjust salt if necessary.
Serve warm.