Place the potatoes in a large pot and cover with water. Bring to a boil, then reduce the heat and simmer for 10–15 minutes, or until fork-tender.
Drain the potatoes well and return them to the pot.
Add the butter, milk, salt, pepper, sour cream, and cream cheese. Mash or mix until smooth and creamy. Avoid overmixing.
Preheat the oven to 350°F.
Spread the mashed potatoes evenly into a 9x9-inch baking dish.
Sprinkle the shredded cheddar cheese evenly over the top.
Cover with foil and bake for 10–15 minutes, or until heated through and the cheese is completely melted.
Top with chopped chives, if desired.
Serve warm.
Notes
For smooth and creamy potatoes, soften the cream cheese before mixing it into the hot potatoes.Avoid overmixing the potatoes, as this can make them gummy.The chives are optional and can be added just before serving.Store leftovers in an airtight container in the refrigerator.