Prepare and boil the potatoes. Fill a saucepan with water and season it with approximately 2 teaspoons of salt. Bring the water to a rolling boil.
While the water heats, cut the cleaned red potatoes into pieces measuring approximately 1 inch so they cook at an even rate.
Carefully lower the potatoes into the boiling water and allow the water to return to a boil. Cook for approximately 15–20 minutes, or until the potatoes are tender enough to pierce easily with a fork.
Drain and season. Drain the cooked potatoes thoroughly, then transfer them to a mixing bowl.
Add the sour cream, garlic powder, and 1 teaspoon of salt. Stir the ingredients together so the seasonings and sour cream begin to coat the warm potatoes evenly.
Mash the potatoes. Using a potato masher, crush the potatoes until they reach the desired consistency. Mash thoroughly for a smoother texture or leave some pieces intact for a more rustic result.
Mix in the grated Parmesan and shredded aged cheddar for additional savory flavor.
Finish and serve. Spoon the mashed red potatoes into a serving dish. Place the butter over the hot potatoes and allow it to melt across the surface.
Serve the mashed potatoes while hot.